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

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.

[1]: https://gitlab.freedesktop.org/xorg/xserver/-/blob/6c51818a0f55282cbe5a870f58ca82ca45ee472d/hw/xwayland/xwayland-glamor-gbm.c#L328



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

unstable/linux-dmabuf/linux-dmabuf-unstable-v1.xml

@@ -143,6 +143,9 @@
         is as if no explicit modifier is specified. The effective modifier
         will be derived from the dmabuf.
 
+        A compositor that sends valid modifiers and DRM_FORMAT_MOD_INVALID for
+        a given format supports both explicit modifiers and implicit modifiers.
+
         For the definition of the format and modifier codes, see the
         zwp_linux_buffer_params_v1::create and zwp_linux_buffer_params_v1::add
         requests.