[PATCH] drm/fourcc: add LINEAR modifiers with an exact pitch alignment

[Marek Olšák]

>
>  * Modifiers must uniquely encode buffer layout. In other words, a buffer
> must
>  * match only a single modifier.
>

That sentence is misleading and impossible to meet. Specifications are
sometimes changed to reflect the overwhelming reality. Multiple modifiers
can represent identical layouts - they already do between vendors, between
generations of the same vendor, and accidentally even between chips of the
same generation. Modifiers have already become 64-bit structures of
bitfields with currently 2^16 possible modifiers for some vendors, and
possibly exceeding 100k for all vendors combined. Encoding all linear
constraints into the 64 bits is one option. It needs more thought, but
encoding at least some constraints in the modifier is not totally off the
table.

The semi-functional LINEAR modifier needs to go. The idea of modifiers is
that nobody should have to expose one that is unsupported to keep things
working for a subset of apps. If the LINEAR modifier is a requirement
everywhere because of apps, and even drivers that can't support it must
expose it, that's a problem. It causes GBM/EGL to fail to import a DMABUF
for a random reason and it can't be prevented without, say, looking at PCI
IDs. If that happened for any other API, it would be considered unusable.
We can either fix it (by replacing/deprecating/removing LINEAR) or abandon
modifiers and replace them with something that works.

Marek
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