Looks like that there actually are another subset of laptops on the market that don't support the Intel HDR backlight interface, but do advertise support for the VESA DPCD backlight interface despite the fact it doesn't seem to work.
Note though I'm not entirely clear on this - on one of the machines where this issue was observed, I also noticed that we appeared to be rejecting the VBT defined backlight frequency in intel_dp_aux_vesa_calc_max_backlight(). It's noted in this function that:
/* Use highest possible value of Pn for more granularity of brightness * adjustment while satifying the conditions below. * ... * - FxP is within 25% of desired value. * Note: 25% is arbitrary value and may need some tweak. */
So it's possible that this value might just need to be tweaked, but for now let's just disable the VESA backlight interface unless it's specified in the VBT just to be safe. We might be able to try enabling this again by default in the future.
Fixes: 2227816e647a ("drm/i915/dp: Allow forcing specific interfaces through enable_dpcd_backlight") Cc: Jani Nikula jani.nikula@intel.com Cc: Rodrigo Vivi rodrigo.vivi@intel.com Bugzilla: https://gitlab.freedesktop.org/drm/intel/-/issues/3169 Signed-off-by: Lyude Paul lyude@redhat.com --- drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c | 1 - 1 file changed, 1 deletion(-)
diff --git a/drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c b/drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c index 651884390137..4f8337c7fd2e 100644 --- a/drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c +++ b/drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c @@ -646,7 +646,6 @@ int intel_dp_aux_init_backlight_funcs(struct intel_connector *connector) break; case INTEL_BACKLIGHT_DISPLAY_DDI: try_intel_interface = true; - try_vesa_interface = true; break; default: return -ENODEV;