Panfrost update: a new kernel driver

The video

Below you can see the same scene that I recorded in January, which was rendered by Panfrost in Mesa but using Arm's kernel driver. This time, Panfrost is using a new kernel driver that is in a form close to be acceptable in the mainline kernel:

The history behind it

During the past two months Rob Herring and I have been working on a new driver for Midgard and Bifrost GPUs that could be accepted mainline.

Arm already maintains a driver out of tree with an acceptable open source license, but it doesn't implement the DRM ABI and several design considerations make it unsuitable for inclusion in mainline Linux.

The absence of a driver in mainline prevents users from keeping their kernels up-to-date and hurts integration with other parts of the free software stack. It also discourages SoC and BSP vendors from submitting their code to mainline, and hurts their ability to track mainline closely.

Besides the code of the driver itself, there's one more condition for mainline inclusion: an open source implementation of the userspace library needs to exist, so other kernel contributors can help verifying, debugging and maintaining the kernel driver. It's an enormous pile of difficult work to reverse engineer the inner workings of a GPU and then implement a compiler and command submission infrastructure, so big thanks to Alyssa Rosenzweig for leading that effort.

Upstream status

Most of the Panfrost code is already part of mainline Mesa, with the code that directly interacts with the new DRM driver being in the review stage. Currently targeted GPUs are T760 and T860, with the RK3399 being the SoC more often used for testing.

The kernel driver is being developed in the open and though we are trying to follow the best practices as displayed by other DRM drivers, there's a number of tasks that need to be done before we consider it ready for submission.

The work ahead

In the kernel:

- Make MMU code more complete for correctness and better performance

- Handle errors and hangs and correctly reset the GPU

- Improve fence handling

- Test with compute shaders (to check completeness of the ABI)

- Lots of cleanups and bug fixing!

In Mesa:

- Get GNOME Shell working

- Get Chromium working with accelerated WebGL

- Get all of glmark2 working

- Get a decent subset of dEQP passing and use it in CI

- Keep refactoring the code

- Support more hardware

Get the code

The exact bits used for the demo recorded above are in various stages of getting upstreamed to the various upstreams, but here are in branches for easier reproduction:

- https://gitlab.freedesktop.org/panfrost/linux/tree/panfrost-5.0-rc4

- https://gitlab.freedesktop.org/tomeu/mesa/tree/mainline-driver