Add support for a DRM backlight capability

At Display Next Hackfest 2025 we discussed the renewed need for moving
brightness control into the DRM connector properties.  I've taken the
previous efforts from David and Marta, rebased and adjusted for the
current kernel.

The legacy sysfs interface is synchronized with the DRM connector (although
the scale may be different as DRM connector property is u16).

Later after this has been adopted by enough userspace, it may make sense to
configure the legacy sysfs interface to be configurable so that only
DRM master controls backlight.

I've done a first implementation with amdgpu with eDP connectors; but
conceivably this can be extended to other connectors like DP for displays
that can be controlled via DDC as well later.

I have also used DRM review prompts to review this series and fix some bugs
which were caught with two different Claude models.  The fixes are squashed
into the patches.

Assisted-by: Claude Opus
Assisted-by: Claude Sonnet

For ease of testing; this series is also available on this branch:
https://git.kernel.org/pub/scm/linux/kernel/git/superm1/l...

---
v3->v4:
 * Address feedback on mailing list for linking, bridge connector handling
 * Make DRM capability block with writes
 * Drop unnecessary code

Mario Limonciello (AMD) (9):
  Revert "backlight: Remove notifier"
  backlight: add kernel-internal backlight API
  drm: link connectors to backlight devices
  DRM: Add support for client and driver indicating support for
    luminance
  drm/amd/display: Pass up errors reading actual brightness
  drm/amd: Indicate driver supports luminance
  drm/amd/display: Allow backlight registration to fail
  drm/amd/display: use drm backlight
  drm/bridge: auto-link panel backlight in bridge connector

 drivers/gpu/drm/Kconfig                       |   1 +
 drivers/gpu/drm/Makefile                      |   2 +
 drivers/gpu/drm/amd/amdgpu/amdgpu_drv.c       |   1 +
 .../gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c |  77 ++-
 drivers/gpu/drm/bridge/panel.c                |  15 +
 .../gpu/drm/display/drm_bridge_connector.c    |  15 +-
 drivers/gpu/drm/drm_atomic_helper.c           |   9 +
 drivers/gpu/drm/drm_atomic_uapi.c             |  59 ++-
 drivers/gpu/drm/drm_backlight.c               | 467 ++++++++++++++++++
 drivers/gpu/drm/drm_connector.c               |  12 +
 drivers/gpu/drm/drm_drv.c                     |   8 +
 drivers/gpu/drm/drm_file.c                    |   5 +
 drivers/gpu/drm/drm_ioctl.c                   |  17 +
 drivers/gpu/drm/drm_mode_config.c             |   7 +
 drivers/gpu/drm/drm_mode_object.c             |  66 ++-
 drivers/gpu/drm/drm_sysfs.c                   |  28 +-
 drivers/video/backlight/backlight.c           |  99 ++++
 include/drm/drm_backlight.h                   |  51 ++
 include/drm/drm_bridge.h                      |   1 +
 include/drm/drm_connector.h                   |   8 +
 include/drm/drm_drv.h                         |   7 +
 include/drm/drm_file.h                        |   8 +
 include/drm/drm_mode_config.h                 |   5 +
 include/linux/backlight.h                     |  56 +++
 include/uapi/drm/drm.h                        |  10 +
 25 files changed, 1005 insertions(+), 29 deletions(-)
 create mode 100644 drivers/gpu/drm/drm_backlight.c
 create mode 100644 include/drm/drm_backlight.h

-- 
2.54.0