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

[Michel Dänzer]

On 2024-12-19 10:02, Daniel Stone wrote:
> 
> How this would be used in practice is also way too underdocumented. We
> need to document that exact-round-up 64b is more restrictive than
> any-multiple-of 64b is more restrictive than 'classic' linear. We need
> to document what people should advertise - if we were starting from
> scratch, the clear answer would be that anything which doesn't care
> should advertise all three, anything advertising any-multiple-of
> should also advertise exact-round-up, etc.
> 
> But we're not starting from scratch, and since linear is 'special',
> userspace already has explicit knowledge of it. So AMD is going to
> have to advertise LINEAR forever, because media frameworks know about
> DRM_FORMAT_MOD_LINEAR and pass that around explicitly when they know
> that the buffer is linear. That and not breaking older userspace
> running in containers or as part of a bisect or whatever.
> 
> There's also the question of what e.g. gbm_bo_get_modifier() should
> return. Again, if we were starting from scratch, most restrictive
> would make sense. But we're not, so I think it has to return LINEAR
> for maximum compatibility (because modifiers can't be morphed into
> other ones for fun), which further cements that we're not removing
> LINEAR.
> 
> And how should allocators determine what to go for? Given that, I
> think the only sensible semantics are, when only LINEAR has been
> passed, to pick the most restrictive set possible; when LINEAR
> variants have been passed as well as LINEAR, to act as if LINEAR were
> not passed at all.

These are all very good points, which call for stricter-than-usual application of the "new UAPI requires corresponding user-space patches" rule:

* Patches adding support for the new modifiers in Mesa (and kernel) drivers for at least two separate vendors
* Patches adding support in at least one non-Mesa user-space component, e.g. a Wayland compositor which has code using the existing linear modifier (e.g. mutter)


Ideally also describe a specific multi-vendor scenario which can't work with the existing linear modifier, and validate that it works with the new ones.


-- 
Earthling Michel Dänzer       \        GNOME / Xwayland / Mesa developer
https://redhat.com             \               Libre software enthusiast