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

[Michel Dänzer]

On 2024-12-16 22:29, Marek Olšák wrote:
> On Mon, Dec 16, 2024 at 4:27 AM Michel Dänzer <michel.daenzer at mailbox.org <mailto: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> <mailto: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.

The comment above the modifier define should spell that out unambiguously.


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