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linux-dmabuf: clarify what mixed valid/INVALID modifiers mean

Simon Ser requested to merge emersion/wayland-protocols:dmabuf-mixed-valid-invalid into master

This commit makes it clear that compositors can send valid modifiers and DRM_FORMAT_MOD_INVALID for a given format. This means that the compositor supports both implicit and explicit modifiers. See the warning further down:

Warning: It should be an error if the format/modifier pair was not advertised with the modifier event. This is not enforced yet because some implementations always accept DRM_FORMAT_MOD_INVALID. Also version 2 of this protocol does not have the modifier event.

Xwayland already requires compositors to send DRM_FORMAT_MOD_INVALID for importing buffers with an implicit modifier 1.

In a future protocol version, it would be nice to make it a protocol error (or at least a soft failure) to use any format/modifier pair that wasn't advertised. A use-case for this is Vulkan compositors: the Vulkan DMA-BUF extensions require an explicit modifier and cannot import buffers which have an implicit modifier.

Signed-off-by: Simon Ser contact@emersion.fr

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