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Alyssa Rosenzweig
@alyssarzg
Linux hacker tinkering with graphics drivers
Joined July 2021
1 Following
15.4K Followers

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Alyssa Rosenzweig
@alyssarzg
No hardware documentation. No reference code. No hardware I can control. No problem. 1 hardware bug only affecting MediaTek. 1 software bug only affecting Apple M1. 1 new reverse-engineering tale. Get the scoop on writing a graphics driver in the dark:
collabora.com
Writing an open source GPU driver - without the hardware
Until now, no Valhall devices (Mali-G57, Mali-G78) ran mainline Linux - whilst this made driver development obviously difficult, there’s no better time to write drivers than before the devices even...
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Alyssa Rosenzweig
@alyssarzg
Now with documentation on the "twiddled" image layout used by the Apple GPU.
@LinaAsahi
,
@dougallj
, hopefully we can get closure now😊 mesa.pages.freedesktop.org/-/mesa/-/jobs/
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Alyssa Rosenzweig
@alyssarzg
·
I just published "ail", a small library for working with Apple GPU image layouts, incorporating @LinaAsahi and @dougallj's research into the GPU's tiled textures. ail replaces buggy code predating their fantastic research. The code fixes two glmark2 scenes and @Inochi2D. 🧄🧄🧄
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Alyssa Rosenzweig
@alyssarzg
Finally, isl (and ail) decouple the logical representation of layouts from the specific way the layout is communicated with the hardware or the API. This should future-proof the code for Vulkan and future Apple GPUs😉
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Alyssa Rosenzweig
@alyssarzg
For testing, isl (and ail) are "leaf" nodes in Mesa. That requires more code overall than embedding the logic inside the driver. However, hardware tiling/compression is now decoupled from API-visible so we can unit test effectively. ail has 1000+ lines of mipmap test data alone.
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Alyssa Rosenzweig
@alyssarzg
ail borrows isl's solution: standardized unit suffixes for most variables. Sure, you could type foo_pixels + bar_samples ...but to the reviewer, it will sound as ungrammatical as "Big Brother is ungood".
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Alyssa Rosenzweig
@alyssarzg
In theory, we need simple dimensional analysis. In practice, we're writing C code (sigh), so the type system can't check our work. If you accidentally add pixels and samples, gcc won't care (they're both integers). Your code is still broken.
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Alyssa Rosenzweig
@alyssarzg
Worse, layout calculations involve many units with subtle differences. Pixels? Samples? Blocks? For some images, those are all the same. Mix them up and you might not notice... but your users will.
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Alyssa Rosenzweig
@alyssarzg
What's so hard about image layouts? The combinatorics. There are too many image types, dimensions, and formats, all multiplied by bespoke combinations of GPU-specific tiling and compression. Hardware-independent conformance tests aren't enough to check correctness.
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Alyssa Rosenzweig
@alyssarzg
If we fix bugs, someone complains the code is too slow. If we make it fast, someone complains it's missing an obscure feature. If we do it all, we burn out. (And someone complains about that.) Be kind to your FOSS maintainers.
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Alyssa Rosenzweig
@alyssarzg
Congrats
@LinaAsahi
for getting kmscube spinning with an open source Linux driver for the Apple M1 GPU* * on an x86 machine over USB, with my Mesa driver and her Python kernel driver. I am proud to be a part of this eldritch horror 🥰
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Alyssa Rosenzweig
@alyssarzg
How To "Just Say No" To Becoming The Primary Maintainer Of Somebody's Abandoned Open Source Project That You Don't Use But Feel Guilty Leaving Unmaintained (For Dummies)
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Alyssa Rosenzweig
@alyssarzg
I'm excited to join the
@XOrgFoundation
board. The X.Org Foundation stewards the free and open source graphics stack. X.Org members' work ranges from Linux's graphics subsystem to Mesa to X and Wayland Thank you for what you do.
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Collabora
@Collabora
·
A big congrats to Alyssa Rosenzweig (@alyssarzg) for being elected to the @XOrgFoundation Board of Directors! She joins Mark Filion, Collabora's Senior Marketing Manager, who was also re-elected for a second term! x.org/wiki/BoardOfDi #Panfrost #OpenSource
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Alyssa Rosenzweig
@alyssarzg
Nothing like working on the M1 Linux GPU driver from my M1 Linux machine :-) 1/2 mesa:asahi / libasahi_tests OK 0.00s 2/2 mesa:asahi / agx_tests OK 0.00s
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Alyssa Rosenzweig
@alyssarzg
Me, playing with Verilog: I have so much respect for hardware engineers. I am humbled... software is easier. Me, writing drivers: What the actual **** were the hardware people thinking? Steaming garbage. how can anyone with multiple brain cells think ANY of this is a good idea?
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Alyssa Rosenzweig
@alyssarzg
Nontrivial data structures by language I use: Python: import nontrivial_data_structure C++: (500 lines of template errors) Rust: Crystal clear error from borrow checker, no idea how to fix, screw it I'll use an arena. C: Segmentation fault
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Alyssa Rosenzweig
@alyssarzg
Anyway, I've had a lot of fun bringing up open source drivers for Mali "Valhall" GPUs. So I hope you enjoy reading about the final product, more than I "enjoyed" debugging two unrelated cache coherency problems to get there 😅
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Alyssa Rosenzweig
@alyssarzg
"Wait, Alyssa, this driver is new Mali GPUs. How could an M1 issue factor in?" Shameless, completely accurate M1 Linux click bait. Look, somehow a quirk of Apple's input/output memory management unit (DART) triggered a bug in *userspace*. Oops?
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