Beyond general review, I'm looking for feedback on a few things specifically here:
-Is the level of backwards compatibility described here sufficient? Technically I can make the user space drivers support the old modifiers too, but that would mean the layout they specify would morph based on the GPU they're being used on, and sharing buffers between two different NV GPUs, which would appear to be possible, would result in corruption on one side or the other.
-I used "magic" numbers for all the bit shifting. Would it be better to use __fourcc_XXX constants like the broadcom modifiers do? I wasn't sure which style was preferred. The nouveau code is full of magic numbers, but that's a bit lower level than this file.
If preferred, I can send this out as part of a patchset that adds support for the modifiers to nouveau and TegraDRM, but I have some things to clean up there before it's ready for proper review, and I didn't want to block review of the basic modifier layout on that work.