Linux 5.12 Could Support Intel's Proprietary HDR Backlight Interface

It didn't land for Linux 5.11 but it looks like Linux 5.12 could end up supporting Intel's "HDR Backlight Interface" for helping newer Intel laptops with their backlight controls where they don't comply with VESA specifications but rather catering to Intel's proprietary interface.

Lyude Paul of Red Hat has been spending some time the past several months working to properly handle Intel's eDP backlight controls used by newer laptops, dubbed the Intel HDR backlight interface as the implementation appears primarily with notebooks using High Dynamic Range panels. Patches by Lyude for this interface have been floating around for several months but not yet merged.

However, giving hope to potential Linux 5.12 support is the HDR backlight interface register definitions being queued last week to drm-intel-next as material for Linux 5.12. Just the register definitions and other work were queued so far without the actual Intel HDR backlight support, but given work is just beginning around new features to ultimately appear in the spring with Linux 5.12 and seeing these early bits queued, it's hopeful we could see the complete implementation ready for the next kernel cycle.

Some newer laptops are not compliant (i.e. broken) with the VESA eDP backlight control interface but rather just properly implementing Intel's DPCD backlight interface. Intel's proprietary backlight interface doesn't appear to be publicly documented or a focus for the company as far as Linux support is concerned, thus leaving Red Hat to figure it out as they work to improve the Linux laptop experience on devices like Lenovo ThinkPad laptops. This proprietary interface appears to be primarily used by Intel powered laptops with HDR display panels.

With some laptops exposing support for both the VESA backlight controls and the Intel HDR backlight interface but only actually providing proper, working support for the latter has resulted in a recent mess. Due to this mess, once the Intel backlight support is merged, that approach will be attempted first prior to falling back to the VESA eDP backlight controls and then DDI backlight controls after that. This will also allow some current device quirks/workarounds to be removed around the VESA eDP backlight handling.

While the register definitions are in drm-intel-next so far, the rest of Lyude's work can for now be found via the FreeDesktop.org patchwork.