AMD Preps HDMI FRL Fixes, New Knob For Disabling Older AMD GPU DCE Display Support

Overnight AMD engineers sent out their latest round of AMDGPU DC display code updates for their Linux driver. This week's round of display code updates include some additional fixes for their recently introduced HDMI FRL (Fixed Rate Link) support as part of the work toward their HDMI 2.1 implementation finally coming together for their open-source, upstream Linux driver. Of the 34 patches posted to the mailing list overnight, there are some HDMI FRL fixes such as for restoring the FRL cap on non-destructive HDMI link verify handling. There is also an update to the FRL link training (LT) timeout behavior. These will hopefully help with some of the corner cases with the experimental HDMI FRL support for those opting into it since at the moment the functionality is disabled by default. These latest AMDGPU DC patches also continue expanding their KUnit test coverage of their different display driver code paths. The KUnit coverage has been a recent push for the AMDGPU driver and nice to see it happen to hopefully catch bugs sooner and avoiding user regressions. Also worth mentioning with these new AMDGPU DC patches is adding the new CONFIG_DRM_AMD_DC_DCE Kconfig option. AMD is now gating the DCE display code behind CONFIG_DRM_AMD_DC_DCE to allow for DCN-only driver builds if desired. The Display Core Engine is in older Radeon GPUs pre-dating the current Display Core Next (DCN) IP. Display Core Next has been used for the better part of the past decade, originally with Raven Ridge and in the APUs and GPUs since that point. With CONFIG_DRM_AMD_DC_DCE, support for the "legacy" DCE ASICs is now made optional albeit the default kernel configuration is keeping it enabled so there isn't dropping of any existing hardware support by default. These latest patches are out for review on the amd-gfx mailing list. Given the timing of these patches hitting the mailing list, they aren't expected to be mainlined until the Linux v7.4 kernel cycle later in the year with the v7.3 merge window quickly approaching.