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

[Simona Vetter <simona.vetter@ffwll.ch>]

On Thu, Dec 19, 2024 at 05:09:33PM +0100, Michel Dänzer wrote:
> 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

I think this is too strict? At least I could come up with other scenarios
where we'd need a new linear variant:
- one driver, but two different devices that happen to have incompatible
  linear requirements which break and get fixed with the new linear mode.
- one driver, one device, but non-driver userspace allocates the linear
  buffer somewhere else (e.g. from dma-buf heaps) and gets pitch
  constraints wrong

> * 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)

This also feels a bit too strict, since I think what Daniel proposed is
that drivers do the special LINEAR handling when there are variants
present in the list of compatible modifiers for an alloation. Hence I
don't think compositor patches are necessarily required, but we definitely
need to test to make sure it actually works and there's not surprises.

The exception is of course when non-mesa userspace allocates/sizes the
buffer itself (maybe for an soc where the display is separate and the gpu
has stricter stride constraints than the display).

> 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.

I think that's really the crucial part, because adding modifiers without
an actual use-case that they fix is just asking for more future trouble I
think.
-Sima

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

-- 
Simona Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch