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

[Marek Olšák]

On Mon, Dec 16, 2024 at 4:27 AM Michel Dänzer <michel.daenzer at mailbox.org>
wrote:

> On 2024-12-15 21:53, Marek Olšák wrote:
> > The comment explains the problem with DRM_FORMAT_MOD_LINEAR.
> >
> > Signed-off-by: Marek Olšák <marek.olsak at amd.com <mailto:
> marek.olsak at 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)
>
> It's not clear what you mean by "requires that the pitch alignment is
> exactly the number in the definition", since a pitch which is aligned to
> 256 bytes is also aligned to 128 & 64 bytes. Do you mean the pitch must be
> exactly the width rounded up to the next / smallest possible multiple of
> the specified number of bytes?
>

The pitch must be width*Bpp aligned to the number of bytes in the
definition.

Marek
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