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Below are the 20 most recent journal entries recorded in Eric Anholt's LiveJournal:

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Tuesday, January 24th, 2017
10:51 am
This week in vc4 (2017-01-24): LCA, DSI, HDMI audio
I spent last week at linux.conf.au 2017 (the best open source conference), mostly talking to people and going to talks instead of acually hacking on vc4.  I'd like to highlight a few talks you could go watch if you weren't there.

Jack Moffit gave an overview of what Servo is doing with Rust on parallel processing and process isolation in web engines.  I've been hacking on this project in my spare time, and the community is delightful to work with.  Check out his talk to see what I've been excited about recently.

I then spent the next session in the hallway chatting with him instead of going to Willy's excellent talk about how the Linux page cache works.  One of the subjects of that talk makes me wonder: I've got these contiguous regions of GPU memory on VC4, could I use huge pages when we're mapping them for uploads to the GPU?  Would they help enough to matter?

Dave Airlie gave a talk about the structure of the Vulkan drivers and his work with Bas on the Radeon Vulkan driver.  It's awesome how quickly these two have been able to turn around a complete Vulkan driver, and it really validates our open source process and the stack we've built in Mesa.

Nathan Egge from Mozilla gave a talk on accelerating JPEG decode using the GPU.  I thought it was going to be harder to do than he made it out to be, so it was pretty cool to see the performance improvements he got.  The only sad part was that it isn't integrated into WebRender yet as far as I can see.

Karen Sandler talked about the issue of what happens to our copyrights when we die, from experience in working with current project maintainers on relicensing efforts.  I think this is a really important talk for anyone who writes copyleft software.  It actually softened my stance on CLAs (!) but is mostly a call to action for "Nobody has a good process yet for dealing with our copyrights after death, let's work on this."

And, finally, I think the most important talk of the conference to me was Daniel Vetter's Maintainers Don't Scale (no video yet, just a blog post).  The proliferation of boutique trees in the kernel with "overworked" maintainers either ignoring or endlessly bikeshedding the code of outside contributors is why I would never work on the Linux kernel if I wasn't paid to do so.  Daniel actually offers a way forward from the current state of maintainership in the kernel, with group maintainership (a familiar model in healthy large projects) to replace DRM's boutique trees and a reinforcement of the cooperative values that I think are why we all work on free software.  I would love to be able to shut down my boutique DRM and ARM platform trees.

In VC4, I did still manage to write a couple of kernel fixes in response to a security report, and rebase DSI on 4.10 and respin some of its patches for resubmission.  I'm still hoping for DSI in 4.11, but as usual it depends on DT maintainer review (no response for over a month).  I also spent a lot of time with Keith talking about ARM cache coherency questions and trying experiments on my NEON tiling code.

While I was mostly gone last week, though, Boris has been working on finish off my HDMI audio branch.  He now has a branch that has a seriously simplified architecture compared to what I'd talked about with larsc originally (*so* much cleaner than what I was modeling on), and he has it actually playing some staticy audio.  He's now debugging what's causing the static, including comparing dumps of raw audio between VC4 and the firmware, along with the register settings of the HDMI and DMA engines.
Monday, January 16th, 2017
9:28 pm
These weeks in vc4 (2017-01-16): NEON acceleration of tiling
Last week I was on vacation, but the week before that I did some more work on figuring out Intel's Mesa CI system, and Mark Janes has started working on some better documentation for it.  I now understand better how the setup is going to work, but haven't made much progress on actually getting a master running yet.

More fun, though, was finally taking a look at optimizing the tiled texture load/store code.  This got started with Patrick Walton tweeting a link to a blog post on clever math for implementing texture tiling, given a couple of assumptions.

As with all GPUs these days, VC4 swizzles textures into a tiled layout so that when a cacheline is loaded from memory, it will cover nearby pixels in the Y direction as well as X, so that you are more likely to get cache hits for your neighboring pixels in drawing.  And the tiling tends to be multiple levels, so that nearby cachelines are also nearby on the screen, reducing DRAM access latency.

For small textures on VC4, we have a single level of tiling: 4x4@32bpp blocks ("utiles") of raster order data, themselves arranged in up to a 4x4 block, and this mode is called LT.  Once things go over that size, we go to T tiling, where we call a 4x4 LT block a subtile, and arrange subtiles either clockwise or counterclockwise order within a 2x2 block (a tile, which at this point is now 1kb of data), and tiles themselves are arranged left-to-right, then right-to-left, then left-to-right again.

The first thing I did was implement the blog post's clever math for LT textures.  One of the nice features of the math trick is that it means I can do partial utile updates finally, because it skips around in the GPU's address space based on the CPU-side pixel coordinate instead of trying to go through GPU address space to update a 4x4 utile at a time.  The downside of doing things this way is that jumping around in GPU address space means that our writes are unlikely to benefit from write combining, which is usually important for getting full write performance to GPU memory.  It turned out, though, that the math was so much more efficient than what I was doing that it was still a win.

However, I found out that the clever math made reads slower.  The problem is that, because we're write-combined, reads are uncached -- each load is a separate bus transaction to main memory.  Switching from my old utile-at-a-time load using memcpy to the new code meant that instead of doing 4 loads using NEON to grab a row at a time, we were now doing 16 loads of 32 bits at a time, for what added up to a 30% performance hit.

Reads *shouldn't* be something that people do much, except that we still have some software fallbacks in X on VC4 ("core" unantialiased text rendering, for example, which I need to write the GLSL 1.20 shaders for), which involve doing a read/modify/write cycle on a texture.  My first attempt at fixing the regression was just adding back a fast path that operates on a utile at a time if things are nicely utile-aligned (they generally are).  However, some forced inlining of functions per cpp that I was doing for the unaligned case meant that the glibc memcpy call now got inlined back to being non-NEON, and the "fast" utile code ended up not helping loads.

Relying on details of glibc's implementation (their tradeoff for when to do NEON loads) and of gcc's implementation (when to go from memcpy calls to inlined 32-bits-at-a-time code) seems like a pretty bad idea, so I decided to finally write the NEON acceleration that Eben and I have talked about several times.

My first hope was that I could load a full cacheline with NEON's VLD.  VLD1 only loads up to 1 "quadword" (16 bytes) at a time, so that doesn't seem like much help.  VLD4 can load 64 bytes like we want, but it also turns AOS data into SOA in the process, and there's no corresponding "SOA-back-to-AOS store 8 or 16 bytes at a time" like we need to do to get things back into the CPU's strided representation.  I tried VLD4+VST4 into a temporary, then doing my old untiling path on the cached temporary, but that still left me a few percent slower on loads than not doing any of this work at all.

Finally, I hit on using the VLDM instruction.  It seems to be intended for stack loads/stores, but we can also use it to get 64 bytes of data in from memory untouched into NEON registers, and then I can use 4 (32bpp) or 8 (8 or 16bpp) VST1s to store it to the CPU side.  With this, we get a 208.256% +/- 7.07029% (n=10) improvement to GetTexImage performance at 1024x1024.  Doing the same NEON code for stores gave a 41.2371% +/- 3.52799% (n=10) improvement, probably mostly due to not calling into memcpy and having it go through its size/alignment-based memcpy path choosing process.

I'm not yet hitting full memory bandwidth, but this should be a noticeable improvement to X, and it'll probably help my piglit test suite runtime as well.  Hopefully I'll get the code polished up and landed next week when I get back from LCA.
Monday, January 2nd, 2017
1:01 pm
This week in vc4 (2017-01-02): thread switching, CI
The 3DMMES test has been thoroughly instruction count limited, so any wins we can get on code generation translate pretty directly into performance gains.  Last week I decided to work on fixing up Jonas's patch to schedule instructions in the delay slots of thread switching, which can save us 3 instructions per texture sample.

Thread switching, you may recall, is a trick in the fragment shader to hide texture fetch latency by cooperatively switching to another fragment shader instance after you request a texture fetch, so that it can make some progress when you'd probably be blocked anyway.  This lets us better occupy the ALUs, at the cost of each shader needing to fit into half of the physical register file.

However, VC4 doesn't cancel instructions in the pipeline when we request a switch (same as for within-shader branching), so 3 more instructions from the current shader get executed.  For my first implementation of thread switching, I just dumped in 3 NOPs after each THRSW to fill the delay slots.  This was still a massive win over not threading, so it was good enough.

Jonas's patch tries to fill the delay slots after we schedule in the thread switch by trying to extract the THRSW signal off of the instruction we scheduled and move it up to 2 instructions before that, and then only add enough NOPs to get us 3 slots filled.  There was a little bug (it re-scheduled the thrsw instruction instead of a NOP in trying to pad out to delay slots), but it basically worked and got us a 1.55% instruction count win on shader-db.

The problem was that he was scheduling ALU operations along with the thrsw, and if the thrsw signal was alone without an ALU operation in it, after moving the thrsw up we'd have a NOP left in the old location.  I wrote a followon patch to fix that: We now only schedule thrsws on their own without ALU operations, insert the THRSW as early as we can, and then add NOPs as necessary to fill remaining delay slots.  This was another 0.41% instruction count win.

This isn't as good as it could be.  Maybe we don't fill the slots as well as we could before choosing to schedule thrsw, but instructions we choose to schedule after that could be fit in.  Those would be tricky because we have to check that they don't write flags or the accumulators (which wouldn't be preserved across the thrsw) or new texture coordinates.  We also don't put the THRSW at any particular point in the timeline between sampler request and results collection.  We might be able to get wins by trying to put thrsw at the lowest-register-pressure point between them, so that fewer things need to be forced into physical regs instead of accumulators.

Those will be projects for later.  It's probably much more important that we figure out how to schedule 2-4 samples with a single thrsw, instead of doing a thrsw per sample like we do today.

The other project last week was starting to build up a plan for vc4 CI.  We've had a few regressions to vc4 in Mesa master because developers (including me!) don't do testing on vc4 hardware on every commit.  The Intel folks have a lovely CI system that does piglit, DEQP, and performance regression testing for them, both tracking Mesa master and doing pre-commit tests of voluntarily submitted branches.  I despaired of the work needed to build something that good, but apparently they've open sourced their configuration, so I'll be trying to replicate that in the next few weeks.  This week I worked on getting the hardware and a plan for where to host it.  I'm looking forward to a bright future of 30-minute test runs on the actual hardware and long-term tracking of performance metrics.  Unfortunately, there are no docs to it and I've never worked with jenkins before, so this is going to be a challenge.

Other things: I submitted a patch to mm/ to shut up the CMA warning we've been suffering from (and patching away downstream) for years, got exactly the expected response ("this dmesg spew in an common path might be useful for somebody debugging something some day, so it should stay there"), so hopefully we can just get Fedora and Debian to patch it out instead.  This is yet another data point in favor of Matthew Garrett's plan of "write kernel patches you need, don't bother submitting them upstream". Started testing a new patch to reduce error return rate on vc4 memory allocatoins on upstream kernel, haven't confirmed that it's working yet.  I also spent a lot of time reviewing tarceri's patches to Mesa preparing for the on-disk shader cache, which should help improve app startup times and reduce memory consumption.
Monday, December 19th, 2016
5:00 pm
This week in VC4 (2016-12-19): DSI, SDTV
Two big successes last week.

One is that Dave Airlie has pulled Boris's VEC (STDV) code for 4.10.  We didn't quite make it for getting the DT changes in time to have full support in 4.10, but the DT changes will be a lot easier to backport than the driver code.

The other is that I finally got the DSI panel working, after 9 months of frustrating development.  It turns out that my DSI transactions to the Toshiba chip aren't working, but if I use I2C to the undocumented Atmel microcontroller to relay to the Toshiba in one of the 4 possible orderings of the register sequence, the panel comes up.  The Toshiba docs say I need to do I2C writes at the beginning of the poweron sequence, but the closed firmware uses DSI transactions for the whole sequence, making me suspicious that the Atmel has already done some of the Toshiba register setup.

I've now submitted the DSI driver, panel, and clock patches after some massive cleanup.  It even comes with proper runtime power management for both the panel and the VC4 DSI module.

The next steps in modesetting land will be to write an input driver for the panel, do any reworks we need from review, and get those chunks merged in time for 4.11.  While I'm working on this, Boris is now looking at my HDMI audio code so that hopefully we can get that working as well.  Eben is going to poke Dom about fixing the VC4driver interop with media decode.  I feel like we're now making rapid progress toward feature parity on the modesetting side of the VC4 driver.
Monday, December 5th, 2016
12:17 pm
This week in vc4 (2016-12-05): SDTV, 3DMMES, HDMI audio, DSI
The Raspberry Pi Foundation recently started contracting with Free Electrons to give me some support on the display side of the stack.  Last week I got to review and release their first big piece of work: Boris Brezillon's code for SDTV support.  I had suggested that we use this as the first project because it should have been small and self contained.  It ended up that we had some clock bugs Boris had to fix, and a bug in my core VC4 CRTC code, but he got a working patch series together shockingly quickly.  He did one respin for a couple more fixes once I had tested it, and it's now out on the list waiting for devicetree maintainer review.  If nothing goes wrong, we should have composite out support in 4.11 (we're probably a week late for 4.10).

On my side, I spent some more time on HDMI audio and the DSI panel.  On the audio side, I'm now emitting the GCP packet for audio mute appropriately (I think), and with some more clocking fixes it's now accepting the audio data at the expected rate.  On the DSI front, I fixed a bit of sequencing and added debugfs for the registers like we have in our other encoders.  There's still no actual audio coming out of HDMI, and only white coming out of the panel.

The DSI situation is making me wish for someone else's panel that I could attach to the connector, so I could see if my bugs are in the Atmel bridge programming or in the DSI driver.

I did some more analysis of 3DMMES's shaders, and improved our code generation, for wins of 0.4%, 1.9%, 1.2%, 2.6%, and 1.2%.  I also experimented with changing the UBO (indirect addressed uniform array) upload path, which showed no effect.  3DMMES's uniform arrays are tiny, though, so it may be a win in some other app later.

I also got a couple of new patches from Jonas Pfeil.  I went through his thread switch delay slots patch, which is pretty close to ready.  He has a similar patch for branching delay slots, though apparently that one isn't showing wins yet in things he's tested.  Perhaps most exciting, though, is that he went and implemented an idea I had dropped on github: replacing our shadow copies of raster textures with a bunch of math in the shader and using general memory loads.  This could potentially fix X performance without a compositor, which we otherwise really don't have a workaround for other than "use a compositor."  It could also improve GL-in-a-window performance: right now all of our DRI surfaces are raster format, because we want to be able to get full screen pageflipping, but that means we do the shadow copy if they aren't fullscreen.  Hopefully this week I'll get a chance to test and review it.
Monday, November 28th, 2016
10:43 am
This week in vc4 (2016-11-28): performance tuning
I missed last week's update, but with the holiday it ended up being a short week anyway.

The multithreaded fragment shaders are now in drm-next and Mesa master.  I think this was the last big win for raw GL performance and we're now down to the level of making 1-2% improvements in our commits.  That is, unless we're supposed to be using double-buffer non-MS mode and the closed driver was just missing that feature.  With the glmark2 comparisons I've done, I'm comfortable with this state, though.  I'm now working on performance comparisons for 3DMMES Taiji, which the HW team often uses as a benchmark.  I spent a day or so trying to get it ported to the closed driver and failed, but I've got it working on the open stack and have written a couple of little performance fixes with it.

The first was just a regression fix from the multithreading patch series, but it was impressive that multithreading hid a 2.9% instruction count penalty and still showed gains.

One of the new fixes I've been working on is folding ALU operations into texture coordinate writes.  This came out of frustration from the instruction selection research I had done the last couple of weeks, where all algorithms seemed like they would still need significant peephole optimization after selection.  I finally said "well, how hard would it be to just finish the big selection problems I know are easily doable with peepholes?" and it wasn't all that bad.  The win came out to about 1% of instructions, with a similar benefit to overall 3DMMES performance (it's shocking how ALU-bound 3DMMES is)

I also started on a branch to jump to the end of the program when all 16 pixels in a thread have been discarded.  This had given me a 7.9% win on GLB2.7 on Intel, so I hoped for similar wins here.  3DMMES seemed like a good candidate for testing, too, with a lot of discards that are followed by reams of code that could be skipped, including texturing.  Initial results didn't seem to show a win, but I haven't actually done any stats on it yet.  I also haven't done the usual "draw red where we were applying the optimization" hack to verify that my code is really working, either.

While I've been working on this, Jonas Pfeil (who originally wrote the multithreading support) has been working on a couple of other projects.  He's been trying to get instructions scheduled into the delay slots of thread switches and branching, which should help reduce any regressions those two features might have caused.  More exciting, he's just posed a branch for doing nearest-filtered raster textures (the primary operation in X11 compositing) using direct memory lookups instead of our shadow-copy fallback.  Hopefully I get a chance to review, test, and merge in the next week or two.

On the kernel side, my branches for 4.10 have been pulled.  We've got ETC1 and multithread FS for 4.10, and a performance win in power management.  I've also been helping out and reviewing Boris Brezillon's work for SDTV output in vc4.  Those patches should be hitting the list this week.
Monday, November 14th, 2016
4:00 pm
This week in vc4 (2016-11-14): Multithreaded fragment shaders
I dropped HDMI audio last week because Jonas Pfeil showed up with a pair of branches to do multithreaded fragment shaders.

Some context for multithreaded fragment shaders: Texture lookups are really slow.  I think we eyeballed them as having a latency of around 20 QPU instructions.  We can hide latency sometimes if there's some unrelated math to be done after the texture coordinate calculation but before using the texture sample.  However, in most cases, you need the texture sample results right away for the next bit of work.

To allow programs to hide that latency, there's a cooperative hyperthreading mode that a fragment shader can opt into.  The shader stays in the bottom half of register space, and before it collects the results of a texture fetch, it issues a thread switch signal, which the hardware will use to run a second fragment shader thread until that one issues its own thread switch.  For the second thread, the top bit of the register addresses gets flipped, so the two threads' states don't overlap (except for the accumulators and the flags, which the shaders have to save and restore).

I had delayed working on this because the full solution was going to be really tricky: queue up as many lookups as we can, then thread switch, then collect all those results before switching again, all while respecting the FIFO sizes.  However, Jonas's huge contribution here was in figuring out that you don't need to be perfect, you can get huge gains by thread switching between each texture lookup and reading its results.

The upshot was a 0-20% performance improvement on glmark2 and a performance hit to only one testcase.  With this we're down to 3 subtests that we're slower on than the closed source driver.  Jonas's kernel patch is out on the list, and I rewrote the Mesa series to expose threading to the register allocator and landed all but the enabling patch (which has to wait on the kernel side getting merged).  Hopefully I'll finish merging it in a week.

In the process of writing multithreading, Jonas noticed that we were scheduling our TLB_Z writes early in the program, which can cut into fragment shader parallelism between the QPUs (of which we have 12) because a TLB_Z write locks the scoreboard for that 2x2 pixel quad.  I merged a patch to Mesa that pushes the TLB_Z down to the bottom, at the cost of a single extra QPU instruction.  Some day we should flip QPU scheduling around so that we pair that TLB_Z up better, and fill our delay slots in thread switches and branching as well.

I also tracked down a major performance issue in Raspbian's desktop using the open driver.  I asked them to use xcompmgr a while back, because readback from the front buffer using the GPU is really slow (The texture unit can't read from raster textures, so we have to reformat them to be readable), and made window dragging unbearable.  However, xcompmgr doesn't unredirect fullscreen windows, so full screen GL apps emit copies (with reformatting!) instead of pageflipping.

It looks like the best choice for Raspbian is going to be using compton, an xcompmgr fork that does pageflipping (you get tear free screen updates!) and unredirection of full screen windows (you get pageflipping directly from GL apps).  I've also opened a bug report on compton with a description of how they could improve their GL drawing for a tiled renderer like the Pi, which could improve its performance for windowed updates significantly.

Simon ran into some trouble with compton, so he hasn't flipped the default yet, but I would encourage anyone running a Raspberry Pi desktop to give it a shot -- the improvement should be massive.

Other things last week: More VCHIQ patch review, merging more to the -next branches for 4.10 (Martin Sperl's thermal driver, at last!), and a shadow texturing bugfix.
Monday, November 7th, 2016
3:11 pm
This week in vc4 (2016-11-07): glmark2, ETC1, power management, HDMI audio
I spent a little bit of time last week on actual 3D work.  I finally figured out what was going wrong with glmark2's terrain demo.  The RCP (reciprocal) instruction on vc4 is a very rough approximation, and in translating GLSL code doing floating point divides (which we have to convert from a / b to a * (1 / b)) we use a Newton-Raphson step to improve our approximation.  However, we forgot to do this for the implicit divide we have to do at the end of your vertex shader, so the triangles in the distance were unstable and bounced around based on the error in the RCP output.

I also finally got around to writing kernel-side validation for ETC1 texture compression.  I should be able to merge support for this to v4.10, which is one of the feature checkboxes we were missing compared to the old driver.  Unfortunately ETC1 doesn't end up being very useful for us in open source stacks, because S3TC has been around a lot longer and supported on more hardware, so there's not much content using ETC that I know of outside of Android land.

I also spent a while fixing up various regressions that had happened in Mesa while I'd been playing in the kernel.  Some day I should work on CI infrastructure on real hardware, but unfortunately Mesa doesn't have any centralized CI infrastructure to plug into.  Intel has a test farm, but not all proposed patches get submitted to it, and it (understandably) doesn't have non-Intel hardware.

In kernel land, I sent out a patch to reduce V3D's overhead due to power management.  We were immediately shutting off the 3D engine when it went idle, even if userland might be expected to submit a new frame shortly thereafter.  This was taking up 1% of the CPU on profiles I was looking at last week, and I've seen it up to 3.5%.  Now we keep the GPU on for at least 40ms ("about two frames plus some slack").  If I'm reading right, the closed driver does immediate powerdown as well, so we may be using slightly more power now, but given that power state transitions themselves cost power, and the CPU overhead costs power, hopefully this works out fine anyway (even though we've never done power measurements for the platform).

I also reviewed more patches from Michael Zoran for vchiq.  It should now (once all reviewed patches land) work on arm64.  We still need to get the vchiq-using drivers like camera and vcsm into staging.

Finally, I made more progress on HDMI audio.  One of the ALSA developers, Lars-Peter Clausen, pointed out that ALSA's design is that userspace would provide the IEC958 subframes by using alsalib's iec958 plugin and a bit ouf asoundrc configuration.  He also provided a ton of help debugging that pipeline.  I rebased and cleaned up into a branch that uses simple-audio-card as my machine driver, so the whole stack is pretty trivial.  Now aplay runs successfully, though no sound has come out.  Next step is to go test that the closed stack actually plays audio on this monitor and capture some register state for debugging.
Monday, October 31st, 2016
2:07 pm
This week in vc4 (2016-10-31): HDMI audio
Last week was spent almost entirely on HDMI audio support.

I had started the VC4-side HDMI setup last week, and got the rest of the stubbed bits filled out this week.

Next was starting to get the audio side connected.  I had originally derived my VC4 side from the Mediatek driver, which makes a child device for the generic "hdmi-codec" DAI codec to attach to.  HDMI-codec is a shared implementation for HDMI encoders that take I2S input, which basically just passes audio setup params into the DRM HDMI encoder, whose job is to actually make I2S signals on wires get routed into HDMI audio packets.  They also have a CPU DAI registered through device tree that moves PCM data from memory onto wires, and a machine driver registered through devicetree that connects the CPU DAI's wires to the HDMI codec DAI's wires.  This is apparently how SoC audio is usually structured.

VC4, on the other hand, doesn't have a separate hardware block for moving PCM data from memory onto wires, it just has the MAI_DATA register in the HDMI block that you put S/PDIF (IEC958) data into using the SoC's generic DMA engine.  So I made VC4's HDMI driver register another a child device with the MAI_DATA register's address passed in, and that child device gets bound by the CPU DAI driver and uses the ASoC generic DMA engine support.  That CPU DAI driver also ends up setting up the machine driver, since I didn't have any other reference to the CPU DAI anywhere.  The glue here is all based on static strings I can't rely on, and is a complete hack for now.

Last, notice how I've used "PCM" talking about a working implementation's audio data and "S/PDIF" data for the thing I have to take in?  That's the sticking point.  The MAI_DATA register wants things that look like the AES3 frames description you can find on wikipedia.  I think the HW is generating my preamble.  In the firmware's HDMI audio implementation, they go through the unpacked PCM data and set the channel status bit in each sample according to the S/PDIF channel status word (the first 2 bytes of which are also documented on wikipedia).   At the same time that they're doing that, they also set the parity bit.

My first thought was "Hey, look, ALSA has SNDRV_PCM_FORMAT_IEC958_SUBFRAME, maybe my codec can say it only takes these S/PDIF frames and everything will work out."  No.  It turns out aplay won't generate them and neither will gstreamer.  To make a usable HDMI audio driver, I'm going to need to generate my IEC958 myself.

Linux is happy to generate my channel status word in memory.  I think the hardware is happy to generate my parity bit.  That just leaves making the CPU DAI write the bits of the channel status word into the subframes as PCM samples come in to the driver, and making sure that the MSB of the PCM audio data is bit 27.  I haven't figured out how to do that quite yet, which is the project for this week.

Work-in-progress, completely non-functional code is here.
Monday, October 24th, 2016
11:32 am
This week in vc4 (2016-10-24): HDMI audio, DSI, simulator, linux-next
The most interesting work I did last week was starting to look into HDMI audio.  I've been putting this off because really I'm a 3D developer, not modesetting, and I'm certainly not an audio developer.  But the work needs to get done, so here I am.

HDMI audio on SOCs looks like it's not terribly hard.  VC4's HDMI driver needs to instantiate a platform device for audio when it gets loaded, and attach a function table to it for calls to be made when starting/stopping audio.  Then I write a sound driver that will bind to the node that VC4 created, accepting a parameter of which register to DMA into.  That sound driver will tell ALSA about what formats it can acccept and actually drive the DMA engine to write audio frames into HDMI.  On the VC4 HDMI side, we do a bit of register setup (clocks, channel mappings, and the HDMI audio infoframe) when the sound driver tells us it wants to start.

It all sounds pretty straightforward so far.  I'm currently at the "lots of typing" stage of development of this.

Second, I did a little poking at the DSI branch again.  I've rebased onto 4.9 and tested an idea I had for how DSI might be failing.  Still no luck.  There's code on drm-vc4-dsi-panel-restart-cleanup-4.9, and the length of that name suggests how long I've been bashing my head against DSI.  I would love for anyone out there with the inclination to do display debug to spend some time looking into it.  I know the history there is terrible, but I haven't prioritized cleanup while trying to get the driver working in the first place.

I also finished the simulator cleanup from last week, found a regression in the major functional improvement in the cleanup, and landed what worked.  Dave Airlie pointed out that he took a different approach for testing with virgl called vtest, and I took a long look at that.  Now that I've got my partial cleanup in place, the vtest-style simulator wrapper looks a lot more doable, and if it doesn't have the major failing that swrast did with visuals then it should actually make vc4 simulation more correct.

I landed a very tiny optimization to Mesa that I wrote while debugging DEQP failures.

Finally, I put together the -next branches for upstream bcm2835 now that 4.9-rc1 is out.  So far I've landed a major cleanup of pinctrl setup in the DT (started by me and finished by Gerd Hoffmann from Red Hat, to whom I'm very grateful) and a little USB setup fix.  I did a lot of review of VCHIQ patches for staging, and sort of rewrote Linus Walleij's RFC patch for getting descriptive names for the GPIOs in lsgpio.
Tuesday, October 18th, 2016
11:50 am
This week in vc4 (2016-10-18): X performance, DEQP, simulation
The most visible change this week is a fix to Mesa for texture upload performance.  A user had reported that selection rectangles in LXDE's file manager were really slow.  I brought up sysprof, and it showed that we were doing uncached reads from the GPU in a situation that should have been entirely write-combined writes.

The bug was that when the size of the texture wasn't aligned to utiles, we were loading the previous contents into the temporary buffer before writing the new texture data in and uploading, even if the full texture was being updated.  I've fixed it to check for when the full texture is being uploaded, and not do the initial download.  This bug was getting it on almost any Cairo vector graphics operation with its Xlib backend, so hopefully this helps a lot of people's desktops.

I also worked on a cleanup of the simulator mode.  I use the closed source simulator regularly as part of my work -- it's fairly accurate to hardware behavior, and allows you to trace what's happening when things go wrong, all with the driver code running like "normal" for apps on my x86 desktop.

However, my simulator support is a little invasive to the driver, replacing vc4 ioctl calls with alternative ioctls to the i915 driver.  I started on a series to make vc4_simulator.c take plain VC4 ioctls and translate them, so that the simulator code is entirely contained.  The last step I think I have is figuring out which level to put the simulator's copying in and out of window system framebuffers at.

Last, I got DEQP's GLES2 tests up and running on Raspberry Pi.  These are approximately equivalent to the official conformance tests.  The initial results were quite good -- 18630/19098 (97.5%) passing when run in the piglit framework.  I found a couple of fixes to be made in glClear() support, one of which will affect all gallium drivers.  Of the remainder, there are a few tests that are failing due to test bugs (we expose extensions that allow features the tests don't expect), but most are failing in register allocation.  For the register allocation failures, I see a relatively quick fix that should reduce register pressure in loops.
Tuesday, October 11th, 2016
1:28 pm
This week in vc4 (2016-10-11): QPU user shaders, Processing performance
Last week I started work on making hello_fft work with the vc4 driver loaded.

hello_fft is a demo program for doing FFTs using the QPUs (the shader core in vc4).  Instead of drawing primitives into a framebuffer like a GL pipeline does, though, it uses the User QPU pipeline, which just hands a uniform stream to a QPU shader instance.

Originally I didn't build a QPU user shader ioctl for vc4 because there's no good way to use this pipeline of the VC4.  The hardware isn't quite capable enough to be exposed as OpenCL or GL compute shaders (some research had been done into this, and the designers' conclusion was that the memory acccess support wasn't quite good enough to be useful).  That leaves you with writing VC4 shaders in raw assembly, which I don't recommend.

The other problem for vc4 exposing user shaders is that, since the GPU sits directly on main memory with no MMU in between, GPU shader programs could access anywhere in system memory.  For 3D, there's no need (in GL 2.1) to do general write back to system memory, so the kernel reads your shader code and rejects shaders if they try to do it.  It also makes sure that you bounds-check all reads through the uniforms or texture sampler.  For QPU user shaders, though, the expected mode of using them is to have the VPM DMA units do general loads and stores, so we'd need new validation support.

If it was just this one demo, we might be willing to lose support for it in the transition to the open driver.  However, some modes of accelerated video decode also use QPU shaders at some stage, so we have to have some sort of solution.

My plan is basically to not do validation and have root-only execution of QPU shaders for now.  The firmware communication driver captures hello_fft's request to have the firmware pass QPU shaders to the V3D hardware, and it redirects it into VC4.  VC4 then maintains the little queue of requests coming in, powers up the hardware, feeds them in, and collects the interrupts from the QPU shaders for when they're done (each job request is required to "mov interrupt, 1" at the end of its last shader to be run).

Dom is now off experimenting in what it will take to redirect video decode's QPU usage onto this code.

The other little project last week was fixing Processing's performance on vc4.  It's one of the few apps that is ported to the closed driver, so it's a likely thing to be compared on.

Unfortunately, Processing was clearing its framebuffers really inefficiently.  For non-tiled hardware it was mostly OK, with just a double-clear of the depth buffer, but for vc4 its repeated glClear()s (rather than a single glClear with all the buffers to be cleared set) were triggering flushes and reloads of the scene, and in one case a render of a full screen quad (and a full screen load before doing so) rather than fast clearing.

The solution was to improve the tracking of what buffers are cleared and whether any primitives have been drawn yet, so that I can coalesce sets of repeated or partial clears together while only updating the colors to be cleared.  There's always a tension in this sort of optimization: Should the GL driver be clever to work around apps behaving badly, or should it push that off to the app developer (more like Vulkan does).  In this case, tiled renderer behavior is sufficiently different from non-tiled renderers, and enough apps will hit this path, that it's well worth it.  For Processing, I saw an improvement of about 10% on the demo I was looking at.

Sadly, Processing won't be a good comparison for open vs closed drivers even after these fixes.  For the closed driver Processing uses EGL, which says that depth buffers become undefined on eglSwapBuffers. In its default mode, though, processing uses GLX, which says that the depth buffer is retained on glXSwapBuffers().  To avoid this extra overhead in its GLX mode, you'd need to use glDiscardFramebufferEXT() right before the swap.  Unfortunately, this isn't connected in gallium yet but shouldn't be hard to do once we have an app that we can test.
Monday, October 3rd, 2016
5:28 pm
This week in vc4 (2016-10-03): HDMI, X testing and cleanups, VCHIQ
Last week Adam Jackson landed my X testing series, so now at least 3 of us pushing code are testing glamor with every commit.  I used the series to test a cleanup of the Render extension related code in glamor, deleting 550 lines of junk.  I did this while starting to work on using GL sampler objects, which should help reduce our pixmap allocation and per-draw Render overhead.   That patch is failing make check so far.

The big project for the week was tackling some of the HDMI bug reports.  I pulled out my bigger monitor that actually takes HDMI input, and started working toward getting it to display all of its modes correctly.  We had an easy bug in the clock driver that broke changing to low resolutions: trying to drive the PLL too slow, instead of driving the PLL in spec and using the divider to get down to the clock we want.  We needed infoframes support to fix purple borders on the screen.  We were doing our interlaced video mode programming pretty much entirely wrong (it's not perfect yet, but 1 of the 3 interlaced modes on my montior displays correctly and they all display something).  Finally, I added support for double-clocked CEA modes (generally the low resolution, low refresh ones in your list.  These are probably particularly interesitng for RPi due to all the people running emulators).

Finally, one of the more exciting things for me was that I got general agreement from gregkh to merge the VCHIQ driver through the staging tree, and he even put the initial patch together for me.  If we add in a couple more small helper drivers, we should be able to merge the V4L2 camera driver, and hopefully get closer to doing a V4L2 video decode driver for upstream.  I've now tested a messy series that at least executes "vcdbg log msg."  Those other drivers will definitely need some cleanups, though.
Monday, September 26th, 2016
2:17 pm
This week in vc4 (2016-09-26): XDC 2016, glamor testing, glamor performance
Last week I spent at XDS 2016 with other graphics stack developers.

I gave a talk on X Server testing and our development model.  Since playing around in the Servo project (and a few other github-centric communities) recently, I've become increasingly convinced that github + CI + automatic merges is the right way to go about software development now.  I got a warmer reception from the other X Server developers than I expected, and I hope to keep working on building out this infrastructure.

I also spent a while with keithp and ajax (other core X developers) talking about how to fix our rendering performance.  We've got two big things hurting vc4 right now: Excessive flushing, and lack of bounds on rendering operations.

The excessive flushing one is pretty easy.  X currently does some of its drawing directly to the front buffer (I think this is bad and we should stop, but that *is* what we're doing today).  With glamor, we do our rendering with GL, so the driver accumulates a command stream over time.  In order for the drawing to the front buffer to show up on time, both for running animations, and for that last character you typed before rendering went idle, we have to periodically flush GL's command stream so that its rendering to the front buffer (if any) shows up.

Right now we just glFlush() every time we might block sending things back to a client, which is really frequent.  For a tiling architecture like vc4, each glFlush() is quite possibly a load and store of the entire screen (8MB).

The solution I started back in August is to rate-limit how often we flush.  When the server wakes up (possibly to render), I arm a timer.  When it fires, I flush and disarm it until the next wakeup.  I set the timer to 5ms as an arbitrary value that was clearly not pretending to be vsync.  The state machine was a little tricky and had a bug it seemed (or at least a bad interaction with x11perf -- it was unclear).  But ajax pointed out that I could easily do better: we have a function in glamor that gets called right before it does any GL operation, meaning that we could use that to arm the timer and not do the arming every time the server wakes up to process input.

Bounding our rendering operations is a bit trickier.  We started doing some of this with keithp's glamor rework: We glScissor() all of our rendering to the pCompositeClip (which is usually the bounds of the current window).  For front buffer rendering in X, this is a big win on vc4 because it means that when you update an uncomposited window I know that you're only loading and storing the area covered by the window, not the entire screen.  That's a big bandwidth savings.

However, the pCompositeClip isn't enough.  As I'm typing we should be bounding the GL rendering around each character (or span of them), so that I only load and store one or two tiles rather than the entire window.  It turns out, though, that you've probably already computed the bounds of the operation in the Damage extension, because you have a compositor running!  Wouldn't it be nice if we could just reuse that old damage computation and reference it?

Keith has started on making this possible: First with the idea of const-ifying all the op arguments so that we could reuse their pointers as a cheap cache key, and now with the idea of just passing along a possibly-initialized rectangle with the bounds of the operation.  If you do compute the bounds, then you pass it down to anything you end up calling.

Between these two, we should get much improved performance on general desktop apps on Raspberry Pi.

Other updates: Landed opt_peephole_sel improvement for glmark2 performance (and probably mupen64plus), added regression testing of glamor to the X Server, fixed a couple of glamor rendering bugs.
Monday, September 19th, 2016
12:48 pm
This week in vc4 (2016-09-19): firmware KMS, apitrace testing

While stuck on an airplane, I put together a repository for apitraces with confirmed good images for driver output.  Combined with the piglit patches I pushed, I now have regression testing of actual apps on vc4 (particularly relevant given that I'm working on optimizing one of those apps!)

The flight was to visit the Raspberry Pi Foundation, with the goal of getting something usable for their distro to switch to the open 3D stack.  There's still a giant pile of KMS work to do (HDMI audio, DSI power management, SDTV support, etc.), and waiting for all of that to be regression-free will be a long time.  The question is: what could we do that would get us 3D, even if KMS isn't ready?

So, I put together a quick branch to expose the firmware's display stack as the KMS display pipeline. It's a filthy hack, and loses us a lot of the important new features that the open stack was going to bring (changing video modes in X, vblank timestamping, power management), but it gets us much closer to the featureset of the previous stack.  Hopefully they'll be switching to it as the default in new installs soon.

In debugging while I was here, Simon found that on his HDMI display the color ramps didn't quite match between closed and open drivers.  After a bit of worrying about gamma ramp behavior, I figured out that it was actually that the monitor was using a CEA mode that requires limited range RGB input.  A patch is now on the list.

Wednesday, September 14th, 2016
6:50 am
This week in vc4 2016-09-13: glmark2 performance work
Last week I spent working on the glmark2 performance issues.  I now have a NIR patch out for the pathological conditionals test (it's now faster than on the old driver), and a branch for job shuffling (+17% and +27% on the two desktop tests).

Here's the basic idea of job shuffling:

We're a tiled renderer, and tiled renderers get their wins from having a Clear at the start of the frame (indicating we don't need to load any previous contents into the tile buffer).  When your frame is done, we flush each tile out to memory.  If you do your clear, start rendering some primitives, and then switch to some other FBO (because you're rendering to a texture that you're planning on texturing from in your next draw to the main FBO), we have to flush out all of those tiles, start rendering to the new FBO, and flush its rendering, and then when you come back to the main FBO and we have to reload your old cleared-and-a-few-draws tiles.

Job shuffling deals with this by separating the single GL command stream into separate jobs per FBO.  When you switch to your temporary FBO, we don't flush the old job, we just set it aside.  To make this work we have to add tracking for which buffers have jobs writing into them (so that if you try to read those from another job, we can go flush the job that wrote it), and which buffers have jobs reading from them (so that if you try to write to them, they can get flushed so that they don't get incorrectly updated contents).

This felt like it should have been harder than it was, and there's a spot where I'm using a really bad data structure I had laying around, but that data structure has been bad news since the driver was imported and it hasn't been high in any profiles yet.  The other tests don't seem to have any problem with the possible increased CPU overhead.

The shuffling branch also unearthed a few bugs related to clearing and blitting in the multisample tests.  Some of the piglit cases involved are fixed, but some will be reporting new piglit "regressions" because the tests are now closer to working correctly (sigh, reftests).

I also started writing documentation for updating the system's X and Mesa stack on Raspbian for one of the Foundation's developers.  It's not polished, and if I was rewriting it I would use modular's build.sh instead of some of what I did there.  But it's there for reference.
Tuesday, September 6th, 2016
5:04 pm
This week in vc4 (2016-09-06): glmark2, X testing, kernel maintaining
Last week I was tasked with making performance comparisons between vc4 and the closed driver possible.  I decided to take glmark2 and port it to dispmanx, and submitted a pull request upstream.  It already worked on X11 on the vc4 driver, and I fixed the drm backend to work as well (though the drm backend has performance problems specific to the glmark2 code).

Looking at glmark2, vc4 has a few bugs.  Terrain has some rendering bugs.  The driver on master took a big performance hit on one of the conditionals tests since the loops support was added, because NIR isn't aggressive enough in flattening if statements.  Some of the tests require that we shuffle rendering jobs to avoid extra frame store/loads.  Finally, we need to use the multithreaded fragment shader mode to hide texture fetching latency on a bunch of tests.  Despite the bugs, results looked good.

(Note: I won't be posting comparisons here.  Comparisons will be left up to the reader on their particular hardware and software stacks).

I'm expecting to get to do some glamor work on vc4 again soon, so I spent some of the time while I was waiting for Raspberry Pi builds working on the X Server's testing infrastructure.  I've previously talked about Travis CI, but for it to be really useful it needs to run our integration tests.  I fixed up the piglit XTS test wrapper to not spuriously fail, made the X Test suite spuriously fail less, and worked with Adam Jackson at Red Hat to fix build issues in XTS.  Finally, I wrote scripts that will, if you have an XTS tree and a piglit tree and build Xvfb, actually run the XTS rendering tests at xserver make check time.

Next steps for xserver testing are to test glamor in a similar fashion, and to integrate this testing into travis-ci and land the travis-ci branch.

Finally, I submitted pull requests to the upstream kernel.  4.8 got some fixes for VC4 3D (merged by Dave), and 4.9 got interlaced vblank timing patches from the Mario Kleiner (not yet merged by Dave) and Raspberry Pi Zero (merged by Florian).
Monday, August 29th, 2016
11:55 am
This week in vc4 (2016-08-29): derivatives, GPU hangs, testing, kernel maintaining
I spent a day or so last week cleaning up @jonasarrow's demo patch for derivatives on vc4.  It had been hanging around on the github issue waiting for a rework due to feedback, and I decided to finally just go and do it.  It unfortunately involved totally rewriting their patches (which I dislike doing, it's always more awesome to have the original submitter get credit), but we now have dFdx()/dFdy() on Mesa master.

I also landed a fix for GPU hangs with 16 vertex attributes (4 or more vec4s, aka glsl-routing in piglit).  I'd been debugging this one for a while, and finally came up with an idea ("what if this FIFO here is a bad idea to use and we should be synchronous with this external unit?"), it worked, and a hardware developer confirmed that the fix was correct.  This one got a huge explanation comment.  I also fixed discards inside of if/loop statements -- generally discards get lowered out of ifs, but if it's in a non-unrolled loop we were doing discards ignoring whether the channel was in the loop.

Thanks to review from Rhys, I landed Mesa's Travis build fixes.  Rhys then used Travis to test out a couple of fixes to i915 and r600.  This is pretty cool, but it just makes me really want to get piglit into Travis so that we can get some actual integration testing in this process.

I got xserver's Travis to the point of running the unit tests, and one of them crashes on CI but not locally.  That's interesting.

The last GPU hang I have in piglit is in glsl-vs-loops.  This week I figured out what's going on, and I hope I'll be able to write about a fix next week.

Finally, I landed Stefan Wahren's Raspberry Pi Zero devicetree for upstream.  If nothing goes wrong, the Zero should be supported in 4.9.
Monday, August 22nd, 2016
1:17 pm
vc4 status update for 2016-08-22: camera, NIR, testing
Last week I finally plugged in the camera module I got a while ago to go take a look at what vc4 needs for displaying camera output.

The surprising answer was "nothing."  vc4 could successfully import RGB dmabufs and display them as planes, even though I had been expecting to need fixes on that front.

However, the bcm2835 v4l camera driver needs a lot of work.  First of all, it doesn't use the proper contiguous memory support in v4l (vb2-dma-contig), and instead asks the firmware to copy from the firmware's contiguous memory into vmalloced kernel memory.  This wastes memory and wastes memory bandwidth, and doesn't give us dma-buf support.

Even more, MMAL (the v4l equivalent that the firmware exposes for driving the hardware) wants to output planar buffers with specific padding.  However, instead of using the multi-plane format support in v4l to expose buffers with that padding, the bcm2835 driver asks the firmware to do another copy from the firmware's planar layout into the old no-padding V4L planar format.

As a user of the V4L api, you're also in trouble because none of these formats have any priority information that I can see: The camera driver says it's equally happy to give you RGB or planar, even though RGB costs an extra copy.  I think properly done today, the camera driver would be exposing multi-plane planar YUV, and giving you a mem2mem adapter that could use MMAL calls to turn the planar YUV into RGB.

For now, I've updated the bug report with links to the demo code and instructions.

I also spent a little bit of time last week finishing off the series to use st/nir in vc4.  I managed to get to no regressions, and landed it today.  It doesn't eliminate TGSI, but it does mean TGSI is gone from the normal GLSL path.

Finally, I got inspired to do some work on testing.  I've been doing some free time work on servo, Mozilla's Rust-based web browser, and their development environment has been a delight as a new developer.  All patch submissions, from core developers or from newbies, go through github pull requests.  When you generate a PR, Travis builds and runs the unit tests on the PR.  Then a core developer reviews the code by adding a "r" comment in the PR or provides feedback.  Once it's reviewed, a bot picks up the pull request, tries merging it to master, then runs the full integration test suite on it.  If the test suite passes, the bot merges it to master, otherwise the bot writes a comment with a link to the build/test logs.

Compare this to Mesa's development process.  You make a patch.  You file it in the issue tracker and it gets utterly ignored.  You complain, and someone tells you you got the process wrong, so you join the mailing list and send your patch (and then get a flood of email until you unsubscribe).  It gets mangled by your email client, and you get told to use git-send-email, so you screw around with that for a while before you get an email that will actually show up in people's inboxes.  Then someone reviews it (hopefully) before it scrolls off the end of their inbox, and then it doesn't get committed anyway because your name was familiar enough that the reviewer thought maybe you had commit access.  Or they do land your patch, and it turns out you hasn't run the integration tests and then people complain at you for not testing.

So, as a first step toward making a process like Mozilla's possible, I put some time into fixing up Travis on Mesa, and building Travis support for the X Server.  If I can get Travis to run piglit and ensure that expected-pass tests don't regress, that at least gives us a documentable path for new developers in these two projects to put their code up on github and get automated testing of the branches they're proposing on the mailing lists.
Monday, August 15th, 2016
9:33 am
vc4 status update for 2016-08-15: DSI panel, Raspbian updates, and docs
Last week I mostly worked on getting the upstream work I and others have done into downstream Raspbian (most of that time unfortunately in setting up another Raspbian development environment, after yet another SD card failed).

However, the most exciting thing for most users is that with the merge of the rpi-4.4.y-dsi-stub-squash branch, the DSI display should now come up by default with the open source driver.  This is unfortunately not a full upstreamable DSI driver, because the closed-source firmware is getting in the way of Linux by stealing our interrupts and then talking to the hardware behind our backs.  To work around the firmware, I never talk to the DSI hardware, and we just replace the HVS display plane configuration on the DSI's output pipe.  This means your display backlight is always on and the DSI link is always running, but better that than no display.

I also transferred the wiki I had made for VC4 over to github.  In doing so, I was pleasantly surprised at how much documentation I wanted to write once I got off of the awful wiki software at freedesktop.  You can find more information on VC4 at my mesa and linux trees.

(Side note, wikis on github are interesting.  When you make your fork, you inherit the wiki of whoever you fork from, and you can do PRs back to their wiki similarly to how you would for the main repo.  So my linux tree has Raspberry Pi's wiki too, and I'm wondering if I want to move all of my wiki over to their tree.  I'm not sure.)

Is there anything that people think should be documented for the vc4 project that isn't there?
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