On Mon, Dec 16, 2024 at 04:58:20PM -0500, Marek Olšák wrote:
> On Mon, Dec 16, 2024 at 9:53 AM Simona Vetter <simona.vetter@xxxxxxxx>
> wrote:
>
> > On Mon, Dec 16, 2024 at 11:46:13AM +0100, Lucas Stach wrote:
> > > Am Montag, dem 16.12.2024 um 10:27 +0100 schrieb Michel Dänzer:
> > > > 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@xxxxxxx <mailto:
> > marek.olsak@xxxxxxx>>
> > > > >
> > > > > 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?
> > >
> > > I guess that's the intention here, as some AMD GPUs apparently have
> > > this limitation that they need an exact aligned pitch.
> > >
> > > If we open the can of worms to overhaul the linear modifier, I think it
> > > would make sense to also add modifiers for the more common restriction,
> > > where the pitch needs to be aligned to a specific granule, but the
> > > engine doesn't care if things get overaligned to a multiple of the
> > > granule. Having both sets would probably make it easier for the reader
> > > to see the difference to the exact pitch modifiers proposed in this
> > > patch.
> >
> > Yeah I think with linear modifiers that sepcificy alignment limitations we
> > need to be _extremely_ precise in what exactly is required, and what not.
> > There's all kinds of hilarious stuff that might be incompatible, and if we
> > don't mind those we're right back to the "everyone agrees on what linear
> > means" falacy.
> >
> > So if pitch must be a multiple of 64, but cannot be a multiple of 128
> > (because the hw does not cope with the padding, then that's important).
> > Other things are alignment constraints on the starting point, and any
> > padding you need to add at the bottom (yeah some hw overscans and gets
> > pissed if there's not memory there). So I think it's best to go overboard
> > here with verbosity.
> >
> > There's also really funny stuff like power-of-two alignment and things
> > like that, but I guess we'll get there if that's ever needed. That's also
> > why I think we don't need to add all possible linear modifiers from the
> > start, unless there's maybe too much confusion with stuff like "exactly
> > 64b aligned pitch" and "at least 64b aligned pitch, but you can add lots
> > of padding if you feel like".
> >
>
> Would it be possible and acceptable to require that the offset alignment is
> always 4K and the slice padding is also always 4K to simplify those
> constraints?
For modifiers in general my take is to try to as closely and exhaustively
as possible document the actual hw requirements, and not try to be clever.
I fear otherwise we'll end up in a case of "works for my case" and then
someone uses modifiers in a novel way and it again falls apart.
But also worst case we'll just deprecate underdefinied modifiers and roll
out new ones (like here), if we've missed a corner case. It happens.
If that means we'll end up with modifiers that are strict subsets you can
just add more modifiers to drivers to make things work. Or if there's
awkward overlaps, we can make a new linear modifier with the common
constraints and roll them to all drivers that want to interop. So
shouldn't be an issue if we encounter interop incompatibilites, I just
feel that trying to worry about this preemptively could likely lead to
more trouble and not actually solve any real ones that will pop up in the
future. Because accurately predicting the future is just too hard.
-Sima
--
Simona Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
