Unfortunately, it turned out that the DPCD is also not a reliable way of probing for DPCD backlight support as some panels will lie and say they have DPCD backlight controls when they don't actually.
So, revert back to the old behavior and add a bunch of EDID-based DP quirks for the panels that we know need this. As you might have already guessed, OUI quirks didn't seem to be a very safe bet for these panels due to them not having their device IDs filled out.
Lyude Paul (3): drm/dp: Introduce EDID-based quirks drm/i915: Force DPCD backlight mode on X1 Extreme 2nd Gen 4K AMOLED panel drm/i915: Force DPCD backlight mode for some Dell CML 2020 panels
drivers/gpu/drm/drm_dp_helper.c | 79 +++++++++++++++++++ drivers/gpu/drm/drm_dp_mst_topology.c | 3 +- .../drm/i915/display/intel_display_types.h | 1 + drivers/gpu/drm/i915/display/intel_dp.c | 11 ++- .../drm/i915/display/intel_dp_aux_backlight.c | 25 +++++- drivers/gpu/drm/i915/display/intel_dp_mst.c | 2 +- drivers/gpu/drm/i915/display/intel_psr.c | 2 +- include/drm/drm_dp_helper.h | 21 ++++- 8 files changed, 130 insertions(+), 14 deletions(-)