I recently got a ThinkPad X1 Extreme 2nd Generation for fixing some issues on, and noticed that out of the box the backlight doesn't work. Along the way of fixing that, I found a few issues with how i915 handles DPCD AUX backlight control and fixed them. Now I've got working backlight controls, hooray!
Note that this patch series enables DPCD backlight controls by default. This time however, we ignore the backlight type advertised in the VBT (unless it's a custom backlight interface that doesn't use the standard VESA interface over AUX) and just trust the DPCD.
Lyude Paul (5): drm/i915: Fix eDP DPCD aux max backlight calculations drm/i915: Assume 100% brightness when not in DPCD control mode drm/i915: Fix DPCD register order in intel_dp_aux_enable_backlight() drm/i915: Don't use VBT for detecting DPCD backlight controls drm/i915: Auto detect DPCD backlight support by default
.../drm/i915/display/intel_display_types.h | 3 + .../drm/i915/display/intel_dp_aux_backlight.c | 175 ++++++++++++------ drivers/gpu/drm/i915/i915_params.c | 2 +- drivers/gpu/drm/i915/i915_params.h | 2 +- 4 files changed, 126 insertions(+), 56 deletions(-)