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

A Panfrost milestone

The video

Below you can see glmark2 running as a Wayland client in Weston, on a NanoPC -T4 (so a RK3399 SoC with a Mali T-864 GPU)). It's much smoother than on the video, which is limited to 5FPS by the webcam.

Weston is running with the DRM backend and the GL renderer.

The history behind it

For more than 10 years, at Collabora we have been happily helping our customers to make the most of their hardware by running free software.

One area some of us have specially enjoyed working on has been open drivers for GPUs, which for a long time have been considered the next frontier in the quest to have a full software platform that companies and individuals can understand, improve and fix without having to ask for permission first.

Something that has saddened me a bit has been our reduced ability to help those customers that for one reason or another had chosen a hardware platform with ARM Mali GPUs, as no open driver was available for those.

While our biggest customers were able to get a high level of support from the vendors in order to have the Mali graphics stack well integrated with the rest of their product, the smaller ones had a much harder time in achieving that level of integration, which manifested in reduced performance, increased power consumption and slipped milestones.

That's why we have been following with great interest the several efforts that aimed to come up with an open driver for GPUs in the Mali family, one similar to those already existing for Qualcomm, NVIDIA and Vivante.

At XDC last year we had the chance of meeting the people involved in the latest effort to develop such a driver: Panfrost. And in the months that followed I made some room in my backlog to come up with a plan to give the effort a boost.

At that point, Panfrost was only able to get its bits in the screen by an elaborate hack that involved copying each frame into a X11 SHM buffer, which besides making the setup of the development environment much more cumbersome, invalidated any performance analysis. It also limited testing to demos such as glmark2.

Due to my previous work on Etnaviv I was already familiar with the abstractions in Mesa for setups in which the display of buffers is performed by a device different from the GPU so it was just a matter of seeing how we could get the kernel driver for the Mali GPU to play well with the rest of the stack.

So during the past month or so I have come up with a proper implementation of the winsys abstraction that makes use of ARM's kernel driver. The result is that now developers have a better base on which to work on the rendering side of things.

By properly creating, exporting and importing buffers, we can now run applications on GBM, from demos such as kmscube and glmark2 to compositors such as Weston, but also big applications such as Kodi. We are also supporting zero-copy display of GPU-rendered clients in Weston.

This should make it much easier to work on the rendering side of things, and work on a proper DRM driver in the mainline kernel can proceed in parallel.

For those interested in joining to the effort, Alyssa has graciously taken the time to update the instructions to build and test Panfrost. You can join us at #panfrost in Freenode and can start sending merge requests to Gitlab.

Thanks to Collabora for sponsoring this work and to Alyssa Rosenzweig and Lyude Paul for their previous work and for answering my questions.