From: Rob Clark <robdclark@xxxxxxxxxxxx> The android userspace treats the display pipeline as a realtime problem. And arguably, if your goal is to not miss frame deadlines (ie. vblank), it is. (See https://lwn.net/Articles/809545/ for the best explaination that I found.) But this presents a problem with using workqueues for non-blocking atomic commit_work(), because the SCHED_FIFO userspace thread(s) can preempt the worker. Which is not really the outcome you want.. once the required fences are scheduled, you want to push the atomic commit down to hw ASAP. But the decision of whether commit_work should be RT or not really depends on what userspace is doing. For a pure CFS userspace display pipeline, commit_work() should remain SCHED_NORMAL. To handle this, convert non-blocking commit_work() to use per-CRTC kthread workers, instead of system_unbound_wq. Per-CRTC workers are used to avoid serializing commits when userspace is using a per-CRTC update loop. A client-cap is introduced so that userspace can opt-in to SCHED_FIFO priority commit work. A potential issue is that since 616d91b68cd ("sched: Remove sched_setscheduler*() EXPORTs") we have limited RT priority levels, meaning that commit_work() ends up running at the same priority level as vblank-work. This shouldn't be a big problem *yet*, due to limited use of vblank-work at this point. And if it could be arranged that vblank-work is scheduled before signaling out-fences and/or sending pageflip events, it could probably work ok to use a single priority level for both commit-work and vblank-work. Rob Clark (3): drm/crtc: Introduce per-crtc kworker drm/atomic: Use kthread worker for nonblocking commits drm: Add a client-cap to set scheduling mode drivers/gpu/drm/drm_atomic_helper.c | 13 ++++++---- drivers/gpu/drm/drm_auth.c | 4 ++++ drivers/gpu/drm/drm_crtc.c | 37 +++++++++++++++++++++++++++++ drivers/gpu/drm/drm_ioctl.c | 13 ++++++++++ include/drm/drm_atomic.h | 31 ++++++++++++++++++++++++ include/drm/drm_crtc.h | 10 ++++++++ include/uapi/drm/drm.h | 13 ++++++++++ 7 files changed, 117 insertions(+), 4 deletions(-) -- 2.26.2
