drm/i915/display: Avoid stale PIPE_SCANLINE values after crtc_enable

Browse source

When a CRTC is moved to a different transcoder (e.g. on DP-MST stream
allocation), PIPE_SCANLINE and PIPE_FRMCNT can return values latched
from the previous transcoder/mode for up to one vblank period after
the new pipe is enabled. The vblank evasion code in
intel_pipe_update_start()/end() then samples a stale or boundary
scanline and the frame counter ticks during the critical section,
producing diagnostic errors of the form:

  [243.348405] xe 0000:00:02.0: [drm] *ERROR* Atomic update failure on
pipe B (start=300 end=301) time 61 us, min 2128, max 2161, scanline
start 1200, end 2165
  [248.536260] xe 0000:00:02.0: [drm] *ERROR* Atomic update failure on
pipe B (start=561 end=562) time 61 us, min 2128, max 2161, scanline
start 2162, end 2167

Here "scanline start 1200" is the vblank_start of a previously
programmed mode on a different transcoder, while "2162" is the current
mode's vblank_start sampled before any real frame has been emitted.
Both indicate a stale read rather than a real evasion miss.

Wait for one vblank after crtc_enable() to give the new transcoder a
chance to start producing live PIPE_SCANLINE/FRMCNT values before any
subsequent atomic commit enters the vblank evasion section. This adds
at most one frame of latency on modeset, which is invisible to users.

Reproduced with igt@kms_rotation_crc@sprite-rotation-180 on a DP-MST
sink; with this patch the failures no longer occur.

Signed-off-by: Suraj Kandpal <suraj.kandpal@intel.com>
Reviewed-by: Pranay Samala <pranay.samala@intel.com>
Link: https://patch.msgid.link/20260429042650.3335718-1-suraj.kandpal@intel.com

...

This commit is contained in:

Suraj Kandpal 2026-04-29 09:56:50 +05:30

parent 5e79af5db0

commit d08e46efae

1 changed files with 2 additions and 0 deletions

Show all changes Ignore whitespace when comparing lines Ignore changes in amount of whitespace Ignore changes in whitespace at EOL

Download patch file Download diff file Expand all files Collapse all files

2

drivers/gpu/drm/i915/display/intel_display.c

View file