Re: [PATCH v7 00/12] Add support for a DRM backlight capability

[Mario Limonciello <mario.limonciello@amd.com>]

On 8/31/26 16:05, Mario Limonciello wrote:
> At Display Next Hackfest 2026 we reviewed progress moving brightness
> control into the DRM connector properties.
> 
> There is a range LUMINANCE property that will default to 0->0.
> Once a driver attaches a backlight it will be updated to 1->max.
> If the panel supports the minimum backlight turning off the display
> the range can later be updated to 0->max instead of 1->max.
> 
> The legacy sysfs interface is synchronized with the DRM connector.
> When a compositor using this feature is loaded, sysfs writes are disabled
> to prevent legacy tools from going out of sync with the compositor.
> 
> This has an implementation initially for amdgpu, i915, and Xe with eDP
> connectors.  It can be extended to other connectors like DP for displays
> that can be controlled via DDC as well later.
> 
> The following compositors have implemented matching support:
>   * Kwin: https://invent.kde.org/plasma/kwin/-/merge_requests/9298
>   * Mutter: https://gitlab.gnome.org/swick/mutter/-/commits/wip/kms-luminance-prop
>   * Wlroots: https://gitlab.freedesktop.org/wlroots/wlroots/-/merge_requests/5384
> 
> v6 -> v7:
>      Core rework (feedback from Maxime Ripard):
>       * Make the DRM backlight core backend-agnostic. Add a struct
>         drm_backlight_funcs indirection so the backlight subsystem is just
>         one backend; other backends (DDC/CI, MIPI-DCS, ...) can be added
>         later without touching the core.
>       * Stop dynamically allocating the DRM backlight. struct drm_backlight
>         is now embedded in struct drm_connector and initialized by the
>         core, so drivers no longer allocate it or handle allocation
>         failure.
>       * Drop the "drm/amd/display: Allow backlight registration to fail"
>         patch; it is no longer needed now that nothing in the alloc path
>         can fail.
>       * Update the LUMINANCE property only through the atomic path. Remove
>         the synchronous backlight writes from the property-set and legacy
>         paths; the hardware is now touched solely from the atomic
>         commit/enable path (via a workqueue, so slow backends never stall a
>         commit).
>       * Move the inlined luminance/DPMS logic into helpers and fix the
>         comment style to match kernel conventions.
> 
>      Per-connector property (fixes a multi-panel bug):
>       * Replace the single device-wide LUMINANCE property with a
>         per-connector property created from the linked backend's range.
>         Previously all connectors shared one property object, so linking a
>         second panel corrupted the range reported for the others.
>       * Never mutate the property min/max after creation (avoids racing
>         GETPROPERTY). Add a kernel-internal drm_property.is_luminance flag
>         to accept value 0 (DPMS off) without a device-wide pointer
>         comparison.
> 
>      Kconfig (feedback from Thomas Zimmermann):
>       * Drop "select BACKLIGHT_CLASS_DEVICE" from DRM. Add config
>         DRM_BACKLIGHT which "depends on" BACKLIGHT_CLASS_DEVICE and
>         provides no-op stubs when disabled, so DRM no longer forces the
>         backlight subsystem into the kernel. Builds verified with the
>         option both enabled and disabled.
> 
>      Documentation (feedback from Hans de Goede):
>       * Document the LUMINANCE range table (1-N vs 0-N), that 1 is the
>         minimum visible brightness, and that 0 means the display is turned
>         off (with its vblank/pageflip implications).
>       * Document BACKLIGHT_UPDATE_DRM.
> 
>      Misc:
>       * backlight: fix a copy/paste kerneldoc on
>         backlight_unregister_notifier and add a struct notifier_block
>         forward declaration in backlight.h.
>       * Split the old "drm: link connectors to backlight devices" patch
>         into "drm/property: add a per-connector luminance flag" and "drm:
>         add connector backlight (LUMINANCE) infrastructure", and rework the
>         capability patch into "drm: add DRM_CLIENT_CAP_LUMINANCE".
>       * Drop all Tested-by tags; the series has changed substantially and
>         needs to be re-tested.
> 
> Mario Limonciello (12):
>    Revert "backlight: Remove notifier"
>    backlight: add kernel-internal backlight API
>    drm/property: add a per-connector luminance flag
>    drm: add connector backlight (LUMINANCE) infrastructure
>    drm: add DRM_CLIENT_CAP_LUMINANCE
>    drm/amd/display: Pass up errors reading actual brightness
>    drm/amd: Indicate driver supports luminance
>    drm/amd/display: use drm backlight
>    drm/bridge: auto-link panel backlight in bridge connector
>    drm/xe: Indicate support for luminance on the connector
>    drm/i915: Indicate support for luminance on the connector
>    drm/i915/display: use drm backlight
> 
>   drivers/gpu/drm/Kconfig                       |  18 +
>   drivers/gpu/drm/Makefile                      |   2 +
>   drivers/gpu/drm/amd/amdgpu/amdgpu_drv.c       |   1 +
>   .../gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c |   5 +-
>   .../display/amdgpu_dm/amdgpu_dm_backlight.c   |  15 +-
>   .../display/amdgpu_dm/amdgpu_dm_backlight.h   |   2 +-
>   .../display/amdgpu_dm/amdgpu_dm_connector.c   |   2 +
>   .../tests/amdgpu_dm_backlight_test.c          |   4 +-
>   drivers/gpu/drm/bridge/panel.c                |  15 +
>   .../gpu/drm/display/drm_bridge_connector.c    |  11 +-
>   drivers/gpu/drm/drm_atomic_helper.c           |  39 ++
>   drivers/gpu/drm/drm_atomic_uapi.c             |  49 +-
>   drivers/gpu/drm/drm_backlight.c               | 515 ++++++++++++++++++
>   drivers/gpu/drm/drm_connector.c               |  56 ++
>   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             |   1 +
>   drivers/gpu/drm/drm_property.c                |   6 +
>   drivers/gpu/drm/drm_sysfs.c                   |  26 +-
>   .../gpu/drm/i915/display/intel_backlight.c    |   4 +
>   drivers/gpu/drm/i915/display/intel_display.c  |   7 +-
>   drivers/gpu/drm/i915/display/intel_dp.c       |   1 +
>   drivers/gpu/drm/i915/i915_driver.c            |   1 +
>   drivers/gpu/drm/xe/xe_device.c                |   3 +-
>   drivers/video/backlight/backlight.c           |  99 ++++
>   include/drm/drm_atomic_helper.h               |   2 +
>   include/drm/drm_backlight.h                   | 158 ++++++
>   include/drm/drm_bridge.h                      |   1 +
>   include/drm/drm_connector.h                   |  20 +
>   include/drm/drm_drv.h                         |  14 +
>   include/drm/drm_file.h                        |   8 +
>   include/drm/drm_property.h                    |  10 +
>   include/linux/backlight.h                     |  63 +++
>   include/uapi/drm/drm.h                        |  22 +
>   35 files changed, 1195 insertions(+), 15 deletions(-)
>   create mode 100644 drivers/gpu/drm/drm_backlight.c
>   create mode 100644 include/drm/drm_backlight.h
> 

I don't think everyone else received it, but there was a good share of 
Sashiko generated feedback from this and a lot of it looks valid.

Don't worry about reviewing v7 of the series.  I'll take into account 
the feedback, re-test and post a v8 when I'm done.