Hello Thomas,
On 3/3/22 21:58, Thomas Zimmermann wrote:
Rework the fbdev deferred-I/O to not interfere with fields of struct page. Make the code more flexible and implement GEM SHMEM mmap on top of it.
This patchset removes the need for a special shadow framebuffer for fbdev mmap when using GEM SHMEM. SHMEM pages are now mmap'ed from /dev/fb directly.
Interesting. I wonder if you have any performance improvements after dropping the shadow buffer.
Patches 2 and 3 rework the fbdev deferred I/O code. It now allows drivers to have better control of the mmap operations. All references to fields in struct page are gone. The rsp state is help in a separate pageref structure.
That's a very nice cleanup. This really was a huge layering violation.
Patches 4 to 7 provide callbacks an helpers to implement deferred I/O with DRM drivers. Specifically, patch 6 introduces a callback to create a dumb buffer for fbdev. This will be useful for many drivers that currently cannot use generic fbdev emulation because of special placement needs of the BO, such as amdgpu or radeon. The drivers can handle the differences to regular dumb buffers in their new callback implementation.
Patch 8 extends the GEM SHMEM memory manager with a new helper for fbdev dumb-buffer creation. The returned BO has it's mmap set up to implement deferred I/O with SHMEM pages. No additional shadow buffer is requires any longer. Many drivers can immediatelly benefit from this change.
Patch 9 extends virtgpu to support fbdev dumb buffers. It's build on top of GEM SHMEM, but has some modifications that need to be implemented for fbdev as well.
There's no immediate fbdev performance improvement from this patchset. Most of all, it removes unnecessary shadow framebuffers and rsp memcpys. A shadow fb for a FullHD display is ~8 MiB, which we now save. The patches do reduce latency between drawing to the fbdev buffer to displaying on the screen. Watching a video on the fbdev console felt smoother and had less flickering.
Awesome. And you also answered here the question I had above.