[Mesa-dev] [PATCH 01/11] drm/amdgpu: Comply with implicit fencing rules

[Rob Clark]

On Fri, May 21, 2021 at 2:10 AM Daniel Vetter <daniel.vetter at ffwll.ch> wrote:
>
> - msm is mildly entertaining. It also supports MSM_SUBMIT_NO_IMPLICIT,
>   but because it doesn't use the drm/scheduler it handles fences from
>   the wrong context with a synchronous dma_fence_wait. See
>   submit_fence_sync() leading to msm_gem_sync_object(). Investing into
>   a scheduler might be a good idea.

Yeah, drm/scheduler is (along with a lot of other things) on the TODO
list.  But this isn't quite as bad as it sounds because userspace uses
a u_queue thread to call the submit ioctl rather than blocking the
driver.  (It also offloads some other work from the driver thread,
like submit merging to reduce # of ioctls.  Coincidentally that
arrangement was a step towards preparing userspace for some
hypothetical non-ioctl uapi ;-))

OTOH it would be good to move blocking until the system can free
enough pages to repin bo's out of the ioctl path to better handle some
memory pressure corner cases without having to be interruptable over a
lot more of the submit path..  Running chrome+android on devices
without a lot of memory is fun..

BR,
-R