Hey,
On 10 December 2014 at 17:17, Rob Clark <robdclark@xxxxxxxxx> wrote:
In DRM/KMS we are lacking a good way to deal with tiled/compressed
formats. Especially in the case of dmabuf/prime buffer sharing, where
we cannot always rely on under-the-hood flags passed to driver specific
gem-create ioctl to pass around these extra flags.
The proposal is to add a per-plane format modifier. This allows to, if
necessary, use different tiling patters for sub-sampled planes, etc.
The format modifiers are added at the end of the ioctl struct, so for
legacy userspace it will be zero padded.
Cool, thanks a lot for looking at this! My one comment is that we could maybe go even further: keep the current modifier as a strict pixel-layout modifier (e.g. tiled, compressed - anything that affects how you actually determine pixel location), and then support an extra (perhaps non-vendor-namespaced) argument for optional pixel _interpretation_ modifiers, e.g. the hints in EGL_EXT_image_dma_buf_import. V4L2 is starting to properly attack things like chroma siting, and being able to specify narrow/wide YUV range is pretty important for STB/DTV in particular. And they're actually starting to move to KMS, too ...
It might be useful to make the interpretation modifiers bitmaskable, so they can be combined (e.g. wide-range/unclamped YUV | whatever chroma siting), but I can't think of a usecase for combining multiple layout modifiers (e.g. this tiling | that compression).
TODO how best to deal with assignment of modifier token values? The
rough idea was to namespace things with an 8bit vendor-id, and then
beyond that it is treated as an opaque value. But that was a relatively
arbitrary choice. There are cases where same tiling pattern and/or
compression is supported by various different vendors. So we should
standardize to use the vendor-id and value of the first one who
documents the format?
Yeah, I'd second all of danvet's comments here, as well as adding a new ADDFB2_MODIFIERS cap + query for supported modifiers. Makes life much easier for generic userspace.
Cheers,
Daniel
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