That was supposed to have Markdown-style section headings, but I forgot that b4 considers a leading # as a comment... sorry for the abrupt topic changes...
The intended headings are below.
On 07/03/2023 23.25, Asahi Lina wrote:
Hi everyone!
This is my first take on the Rust abstractions for the DRM subsystem. It includes the abstractions themselves, some minor prerequisite changes to the C side, as well as the drm-asahi GPU driver (for reference on how the abstractions are used, but not necessarily intended to land together).
These patches apply on top of the tree at [1], which is based on 6.3-rc1 with a large number of Rust abstraction/support commits added on top. Most of these are not prerequisites for the DRM abstractions themselves, but rather only of the driver.
- #1-12 introduce the abstractions, module by module, with minor C changes before the dependent abstraction.
- Patch 10 is a little addition to drm_sched that I ended up needing, but I can pull it out of the abstraction into its own patch if needed.
- #13-14 add a minor feature to drm/gem and its abstraction used by the driver.
- #15-16 introduce the (unstable) asahi UAPI. This is obviously not ready for merge yet, but comments are welcome!
- #17 adds a Rust helper macro to handle GPU core/firmware differences. This probably belongs in the driver at this point, but right now it has to live in rust/macros since there is no mechanism for per-driver proc macros.
- #18 adds the driver proper, in one big commit, for reference purposes.
## Background
I've been working since mid last year on an Apple AGX GPU driver for Linux, using the (at the time) out-of-tree Rust support. As part of this effort, I've been writing safe Rust abstractions for portions of the DRM subsystem.
Now that Rust itself is upstream, I'd like to get all the abstractions upstreamed so we can eventually get the driver upstreamed!
These abstractions have been used by the driver since our release in December [2], in a simpler synchronous-submission form:
- drm::ioctl
- drm::device
- drm::drv
- drm::file
- drm::{gem, gem::shmem}
- drm::mm
This series adds these too, which are used by the explicit sync refactor of the driver (the version in this series):
- drm::syncobj
- drm::sched
- dma_fence
The major dependencies for the DRM abstractions themselves are:
- [3] rust: error: Add missing wrappers to convert to/from kernel error codes
- [4] rust: Miscellaneous macro improvements
- [5] rust: Add a Sealed trait
- [6] rust: device: Add a minimal RawDevice trait
- [7] rust: Enable the new_uninit feature for kernel and driver crates
- [8] rust: ioctl: Add ioctl number manipulation functions
- [9] rust: sync: Arc: Any downcasting and assume_init()
rust: Add `container_of` and `offset_of` macroskernel::sync::mutex and dependenciesMost of these (the ones with links) have already been submitted, and I expect all of them to land for 6.4 (the mutex one will likely be last, since there is some refactoring that will happen over the current state to make it more ergonomic to use). The mutex dep is only necessary for drm::mm and dma_fence, and transitively drm::syncobj and drm::sched.
## State
Things work! We've had most of the abstractions in production edge kernels with the driver, and the new explicit sync stuff has passed quite a few torture tests (this is how we found the drm_sched issue, patch 11).
The abstractions are intended to be safe (safety review very welcome!). While writing them, I tried to avoid making any changes to the C side unless absolutely necessary. I understand that it will probably make sense to adjust the C side to make some things easier, but I wanted to start from this as a baseline.
Known issues:
The existing Rust integration does not currently allow building abstractions as modules, so the Rust abstractions are only available for DRM components that are built in. I added some extra Kconfig symbols to deal with this, so a driver built as a module can depende on having those built in. This should go away in the future (but may not be ready in time for submission... I understand this probably shouldn't be a blocker though?).
DRM relies heavily on the "subclassing" pattern for driver objects, and this doesn't map well to Rust. I tried several approaches for various bits, so we can see how they work out. In particular, whether wrapper types should pretend to be smart pointers and Deref to their inner driver-specific types, and whether they should be marked as method receivers (Yuck, internal rustc implementation hacks! But Arc<T> already does the same thing and it makes usage in driver-implemented callbacks as `self` possible) are things I'd love to discuss ^^.
Only what I need for my driver is implemented (plus a small amount of obvious extras where better API completeness makes sense). I think the general idea with Rust abstractions is that we add things as they become necessary.
The plain GEM vs. GEM-shmem duality ended up with quite a hairy type hierarchy. I'd love to figure out how to make this simpler...
drm::mm ends up requiring a built-in mutex in the abstraction, instead of delegating that to the user with the usual Rust mutability rules. This is because nodes can be dropped at any time, and those operations need to be synchronized. We could try to avoid forbidding those drops or mark the node type !Send, but that would make it a lot less ergonomic to use...
I'm looking for feedback on the abstractions of all kinds, so we can move towards an upstreamable version. Optimistically, I'd love to get this upstream for 6.5, and the driver for 6.6.
Please feel free to ask any questions about the Rust bits, since I know a lot of this is new to many of the C folks!
## About the drm-asahi driver
This is a fairly complete driver for Apple AGX G13 and G14 series GPUs.
The driver today supports the Apple M1, M1 Pro, M1 Max, M1 Ultra, and M2 SoCs, across two firmware revisions each. It has an explicit sync UAPI heavily inspired by the upcoming Intel Xe UAPI, designed with Vulkan support in mind. On the Mesa side we currently have a Gallium driver that is mostly already upstream (missing the UAPI bits mostly) and passes the dEQP GLES2/EGL tests, with most of GLES3.0 passing in downstream work-in-progress branches. This is a reverse engineered community driver (we have no hardware documentation of any kind, other than some hints from aspects shared with PowerVR).
While developing the driver, I tried to make use of Rust's safety and lifetime features to provide not just CPU-side safety, but also partial firmware-ABI safety. Thanks to this, it has turned out to be a very stable driver even though GPU firmware crashes are fatal (no restart capability, need to reboot!) and the FW/driver interface is a huge mess of unsafe shared memory structures with complex pointer chains. There are over 70 ABI types and 3000+ lines of firmware ABI type definitions that vary between firmware builds and GPU cores...
In a simpler blocking-submission form, it has been shipping in Asahi Linux edge kernels since December [2], with lots of users and zero (!) reported oopses (and only a couple reports of GPU firmware crashes, though that issue should now be fixed). It has survived OOM scenarios (Rust makes error cleanup easy!), UAPI-level fuzzing, countless broken Mesa builds, uptimes of 40+ days, and more.
The explicit sync refactor significantly increases performance (and potential problems), but this version has survived a lot of torture with dEQP/piglit tests and some manual corner case testing.
In other words, Rust works! ^^
There are some design notes on the driver and further links at [10].
## Links
[1] https://github.com/AsahiLinux/linux.git drm-rfc-base-20230307 [2] https://asahilinux.org/2022/12/gpu-drivers-now-in-asahi-linux/ [3] https://lore.kernel.org/rust-for-linux/20230224-rust-error-v1-0-f8f9a9a87303... [4] https://lore.kernel.org/rust-for-linux/20230224-rust-macros-v1-0-b39fae46e10... [5] https://lore.kernel.org/rust-for-linux/20230224-rust-iopt-rtkit-v1-0-49ced33... [6] https://lore.kernel.org/rust-for-linux/20230224-rust-iopt-rtkit-v1-0-49ced33... [7] https://lore.kernel.org/rust-for-linux/CQV7ZNT6LMXI.1XG4YXSH8I7JK@vincent-ar... [8] https://lore.kernel.org/rust-for-linux/61f734d6-1497-755f-3632-3f261b890846@... [9] https://lore.kernel.org/rust-for-linux/20230224-rust-arc-v1-0-568eea613a41@a... [10] https://github.com/AsahiLinux/docs/wiki/SW:AGX-driver-notes
~~ Lina