Some context from the article I found helpful as a layman :)

When seeing this mailing list post, my immediate assumption was it being some new HDR testing code that slipped under my radar from DRM-Next coverage, given all the ongoing High Dynamic Range / color management work happening recently for the Linux desktop. But when digging into the merge, it's actually even more mundane. The "hdr" in this sense is around the C header files. The new "hdrtest" code is for the Intel Xe kernel driver and is around trying to help ensure the DRM header files are self-contained and pass kernel-doc tests. Basic maintenance checks on the included DRM header files to ensure they are all in good shape.

Apparently this is about neither DRM, nor HDR in the sense an average user might think of those terms 😅