[PATCH 0/5] drm/amd/display: Commit panel_power_savings writes from the kernel

[Hugo Sales]

A write to panel_power_savings stores the ABM level and sends a hotplug
uevent, but the level reaches the hardware only on the next atomic commit
that includes the eDP connector. Hyprland adds the connector to a commit
only on a modeset or when a connector property it sets changes, so on my
laptop the file read back the new level and the backlight never changed.
power-profiles-daemon writes this file on every profile and AC change, so
none of its ABM changes applied

This shouldn't need a fix in every compositor. The file is for writers
that aren't the DRM master, like power-profiles-daemon, and they can't
commit anything themselves. Today they depend on the master taking a
generic hotplug uevent as a cue to commit the connector again. Nothing
documents that, and a compositor has no reason to read a hotplug event as
"a property you never touched has changed". A compositor that wants to own
ABM sets the 'adaptive backlight modulation' property, and from then on
the file returns -EBUSY. The file has to work without the master's help.
The backlight device already does: a brightness write goes straight to DC,
with no commit at all

This series makes the write commit the level itself:

  1. Render the existing kernel-doc, which no rst file included
  2. Stop forcing a modeset when the connector's ABM level changes. DM
     already sends a new level as a stream update, and the modeset flag
     dates from before the CRTC was added to such commits
  3. Move the store tests onto a full pipe the store can commit to
  4. Commit the level from the store through a duplicated connector
     state. The write returns the commit's error and no longer sends a
     uevent
  5. KUnit cases for a connector with the CRTC on, off and failing
     atomic check

Patch 2 also asks, below its cut, whether scaling can leave the same
check

v1 of the patch that added this file had no flush [1], and Mario found
that the level only took effect on the next modeset [2]. v2's changelog
says "force an atomic commit" [3], but the merged code (63d0b87213a0)
sends a hotplug event instead. 8d20a066fa9b later applied the level
outside atomic and was reverted in 95aaa207e9ef because it broke igt
abm_gradual. The objections back then were to sysfs overriding the DRM
master at all [4], which the property above now covers. I didn't find
any discussion that rejects an internal atomic commit

A write now returns the commit's error instead of storing a level the
hardware may never get. power-profiles-daemon logs the error and tries
again on its next profile, AC or battery change. Nothing I found reads
the uevent for more than a cue to commit the connector, which the kernel
now does itself

The commit is blocking, so a write can hold the CRTC locks for up to a
frame while a compositor commit waits

Tested on my laptop (ASUS ROG Flow X13 GV301QH, DCN 2.1) with the series
on 7.3:

  - igt amd_abm passes all five subtests
  - Under Hyprland, with no modeset during the run, 25 writes cycling
    levels 0 to 4 all succeeded and read back, with no -EBUSY and nothing
    in the kernel log. A write sends no uevent
  - The backlight PWM follows each write. At level 4 the target PWM is
    43% of level 0 at the same brightness, and level 0 restores it

[1] https://lore.kernel.org/amd-gfx/20240126222300.119292-1-hamza.mahfooz@xxxxxxx/
[2] https://lore.kernel.org/amd-gfx/1162e59e-3f53-4c84-b186-bb2f4642216b@xxxxxxx/
[3] https://lore.kernel.org/amd-gfx/20240202152837.7388-1-hamza.mahfooz@xxxxxxx/
[4] https://lore.kernel.org/amd-gfx/b117ccc0-6569-4753-9cb9-c304f9e50d30@xxxxxxx/

Signed-off-by: Hugo Sales <hugo@xxxxxxx>
---
Hugo Sales (5):
      drm/amd/display: Render the panel power savings kernel-doc
      drm/amd/display: Don't force a modeset when the ABM level changes
      drm/amd/display: Give the panel_power_savings store tests a full pipe
      drm/amd/display: Commit panel_power_savings writes from the kernel
      drm/amd/display: Test panel_power_savings commits

 .../gpu/amdgpu/display/display-manager.rst         |   3 +
 drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c  |   3 +-
 .../amd/display/amdgpu_dm/amdgpu_dm_backlight.c    |  90 +++++++-
 .../amdgpu_dm/tests/amdgpu_dm_backlight_test.c     | 231 ++++++++++++++++++++-
 4 files changed, 308 insertions(+), 19 deletions(-)
---
base-commit: ba496bb837882eb1af58f41572fc289e46c77b8c
change-id: 20261101-abm-sysfs-commit-96a38de1ac66

Best regards,
--  
Hugo Sales <hugo@xxxxxxx>