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

[Laurent Pinchart <laurent.pinchart@ideasonboard.com>]

On Tue, Dec 17, 2024 at 11:13:05AM +0100, Michel Dänzer wrote:
> On 2024-12-17 10:14, Brian Starkey wrote:
> > On Sun, Dec 15, 2024 at 03:53:14PM +0000, Marek Olšák wrote:
> >> The comment explains the problem with DRM_FORMAT_MOD_LINEAR.
> >>
> >> Signed-off-by: Marek Olšák <marek.olsak@amd.com>
> >>
> >> diff --git a/include/uapi/drm/drm_fourcc.h b/include/uapi/drm/drm_fourcc.h
> >> index 78abd819fd62e..8ec4163429014 100644
> >> --- a/include/uapi/drm/drm_fourcc.h
> >> +++ b/include/uapi/drm/drm_fourcc.h
> >> @@ -484,9 +484,27 @@ extern "C" {
> >>   * modifier (e.g. not setting DRM_MODE_FB_MODIFIERS in the DRM_ADDFB2 ioctl),
> >>   * which tells the driver to also take driver-internal information into account
> >>   * and so might actually result in a tiled framebuffer.
> >> + *
> >> + * WARNING:
> >> + * There are drivers out there that expose DRM_FORMAT_MOD_LINEAR, but only
> >> + * support a certain pitch alignment and can't import images with this modifier
> >> + * if the pitch alignment isn't exactly the one supported. They can however
> >> + * allocate images with this modifier and other drivers can import them only
> >> + * if they support the same pitch alignment. Thus, DRM_FORMAT_MOD_LINEAR is
> >> + * fundamentically incompatible across devices and is the only modifier that
> >> + * has a chance of not working. The PITCH_ALIGN modifiers should be used
> >> + * instead.
> >>   */
> >>  #define DRM_FORMAT_MOD_LINEAR  fourcc_mod_code(NONE, 0)
> >>
> >> +/* Linear layout modifiers with an explicit pitch alignment in bytes.
> >> + * Exposing this modifier requires that the pitch alignment is exactly
> >> + * the number in the definition.
> >> + */
> >> +#define DRM_FORMAT_MOD_LINEAR_PITCH_ALIGN_64B fourcc_mod_code(NONE, 1)
> > 
> > Why do we want this to be a modifier? All (?) of the other modifiers
> > describe properties which the producer and consumer need to know in
> > order to correctly fill/interpret the data.
> > 
> > Framebuffers already have a pitch property which tells the
> > producer/consumer how to do that for linear buffers.
> 
> At this point, the entity which allocates a linear buffer on device A
> to be shared with another device B can't know the pitch restrictions
> of B. If it guesses incorrectly, accessing the buffer with B won't
> work, so any effort allocating the buffer and producing its contents
> will be wasted.
> 
> > Modifiers are meant to describe framebuffers, and this pitch alignment
> > requirement isn't really a framebuffer property - it's a device
> > constraint. It feels out of place to overload modifiers with it.
> > 
> > I'm not saying we don't need a way to describe constraints to
> > allocators, but I question if modifiers the right mechanism to
> > communicate them?
>
> While I agree with your concern in general, AFAIK there's no other
> solution for this even on the horizon, after years of talking about
> it. The solution proposed here seems like an acceptable stop gap,
> assuming it won't result in a gazillion linear modifiers.

Flipping that argument, the reason why we still have no solution is
because we've constantly accepted stop-gap measures. Maybe it's time to
stop. It may feel a bit unfair to Marek that everybody until know got
away with hacks, but I don't think he would be left alone designing a
proper solution.

-- 
Regards,

Laurent Pinchart