linux-kernel - [PATCH v2 0/3] drm: commit

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Date:   Wed, 30 Sep 2020 14:17:19 -0700
From:   Rob Clark <robdclark@...il.com>
To:     dri-devel@...ts.freedesktop.org
Cc:     linux-arm-msm@...r.kernel.org, Tejun Heo <tj@...nel.org>,
        timmurray@...gle.com, Daniel Vetter <daniel@...ll.ch>,
        Qais Yousef <qais.yousef@....com>,
        Rob Clark <robdclark@...omium.org>,
        linux-kernel@...r.kernel.org (open list)
Subject: [PATCH v2 0/3] drm: commit_work scheduling

From: Rob Clark <robdclark@...omium.org>

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.  And the last patch exposes the task id to userspace as
a CRTC property, so that userspace can adjust the priority and sched
policy to fit it's needs.

v2: Drop client cap and in-kernel setting of priority/policy in
    favor of exposing the kworker tid to userspace so that user-
    space can set priority/policy.

Rob Clark (3):
  drm/crtc: Introduce per-crtc kworker
  drm/atomic: Use kthread worker for nonblocking commits
  drm: Expose CRTC's kworker task id

 drivers/gpu/drm/drm_atomic_helper.c | 13 ++++++++----
 drivers/gpu/drm/drm_crtc.c          | 14 +++++++++++++
 drivers/gpu/drm/drm_mode_config.c   | 14 +++++++++++++
 drivers/gpu/drm/drm_mode_object.c   |  4 ++++
 include/drm/drm_atomic.h            | 31 +++++++++++++++++++++++++++++
 include/drm/drm_crtc.h              |  8 ++++++++
 include/drm/drm_mode_config.h       |  9 +++++++++
 include/drm/drm_property.h          |  9 +++++++++
 8 files changed, 98 insertions(+), 4 deletions(-)

-- 
2.26.2