When I originally moved all of the VESA backlight code in i915 into DRM helpers, one of the things I didn't have the hardware or time for testing was machines that used a combination of PWM and DPCD in order to control their backlights. This has since then caused some breakages and resulted in us disabling DPCD backlight support on such machines. This works fine, unless you have a machine that actually needs this functionality for backlight controls to work at all. Additionally, we will need to support PWM for when we start adding support for VESA's product (as in the product of multiplication) control mode for better brightness ranges.
So - let's finally finish up implementing basic support for these types of backlights to solve these problems in our DP helpers, along with implementing support for this in i915. And since digging into this issue solved the last questions we really had about probing backlights in i915 for the most part, let's update some of the comments around that as well!
Lyude Paul (5): drm/i915: Add support for panels with VESA backlights with PWM enable/disable drm/nouveau/kms/nv50-: Explicitly check DPCD backlights for aux enable/brightness drm/dp: Don't read back backlight mode in drm_edp_backlight_enable() drm/dp, drm/i915: Add support for VESA backlights using PWM for brightness control drm/i915: Clarify probing order in intel_dp_aux_init_backlight_funcs()
drivers/gpu/drm/drm_dp_helper.c | 108 ++++++++++-------- .../drm/i915/display/intel_dp_aux_backlight.c | 81 ++++++++++--- drivers/gpu/drm/nouveau/nouveau_backlight.c | 5 +- include/drm/drm_dp_helper.h | 7 +- 4 files changed, 132 insertions(+), 69 deletions(-)