Intel Improving Its Content Match Refresh Rate "CMRR" Feature With Linux 7.4

Introduced with the Xe2 Lunar Lake graphics hardware was support for Content Match Refresh Rate (CMRR) as an improvement over typical Variable Refresh Rate (VRR) / Adaptive-Sync to more precisely match the video or content frame rates. Particularly for video playback it aims for no stuttering or frame drops. With the upcoming Linux 7.4 kernel will be better support for CMRR with the Intel graphics driver. Back in 2023 is when I was first to report on the new Intel CMRR functionality being worked on by Intel for their open-source graphics driver ahead of the Lunar Lake launch. While Lunar Lake and now Panther Lake and Wildcat Lake have since appeared with Intel hardware continuing to advance and Intel CMRR patches existing for nearly two years, Content Match Refresh Rate hasn't been fully utilized yet under Linux. Only now are patches being queued up to DRM-Next in advance of Linux 7.4 to expand its use. Intel engineer Mitul Golani noted in a patch series to-be-merged for Linux 7.4 that the CMRR fractional-timing code had been permanently disabled, was embedded DisplayPort (eDP) only, and relied on a heuristic to guess when the fractional timing path was needed. The new patches address the shortcomings plus add a new DebugFS-based feature for now in advance of a new user-space API for controlling CMRR. While the CMRR support was introduced with Lunar Lake, the work for Linux 7.4 is default enabling it for cases where the VRR timing generated is the default timing generator, which as of now is only for Intel Display IP version 30 and newer. That corresponds to Xe3 / Panther Lake and newer. The new DebugFS control is a per-CRTC DebugFS file that lets user-space for a CMRR target refresh rate and video mode requirement. The requested refresh rate can be specified in milli-hz andalso specifying a denominator for the video timing. Eventually a nice user-space API is expected but for now it's an Intel driver specific DebugFS interface. This patch series has that latest Intel CMRR enablement work. In turn those patches were sent out via today's drm-intel-next pull request of new Intel kernel graphics driver material destined for Linux 7.4. That pull also adapts DG2 multi-protocol PLL code to using the DPLL framework, DP-HDMI PCON Display Stream Compression fixes, and a variety of other code clean-ups and fixes. There still are several more weeks for additional Intel graphics driver features to land in DRM-Next ahead of the Linux 7.4 merge window in late October.