Re: [PATCH 1/2] drm/nouveau: Add DRM_IOCTL_NOUVEAU_GET_ZCULL_INFO

[Danilo Krummrich <dakr@kernel.org>]

(CC: Ben, John)

On Thu, Mar 27, 2025 at 02:03:21PM -0400, M Henning wrote:
> On Thu, Mar 27, 2025 at 9:51 AM Danilo Krummrich <dakr@kernel.org> wrote:
> >
> > On Fri, Mar 21, 2025 at 06:06:34PM -0400, M Henning wrote:
> > > On Thu, Mar 20, 2025 at 2:18 PM Danilo Krummrich <dakr@kernel.org> wrote:
> > > > On Wed, Mar 12, 2025 at 05:36:14PM -0400, Mel Henning wrote:
> > > > > +     __u32 width_align_pixels;
> > > > > +     __u32 height_align_pixels;
> > > > > +     __u32 pixel_squares_by_aliquots;
> > > > > +     __u32 aliquot_total;
> > > > > +     __u32 zcull_region_byte_multiplier;
> > > > > +     __u32 zcull_region_header_size;
> > > > > +     __u32 zcull_subregion_header_size;
> > > > > +     __u32 subregion_count;
> > > > > +     __u32 subregion_width_align_pixels;
> > > > > +     __u32 subregion_height_align_pixels;
> > > > > +
> > > > > +     __u32 ctxsw_size;
> > > > > +     __u32 ctxsw_align;
> > > > > +};
> > > >
> > > > What if this ever changes between hardware revisions or firmware versions?
> > >
> > > There was some previous discussion of that here:
> > > https://gitlab.freedesktop.org/mesa/mesa/-/issues/12596#note_2796853
> > >
> > > From what I can tell, this structure hasn't really changed since
> > > FERMI_C (circa 2011), so I'm not too worried about it changing on us
> > > too quickly. When it does change, we have the option of appending more
> > > members to this struct in the usual way, or if the change is more
> > > fundamental we can return an error from this ioctl and add a new
> > > interface. Userspace needs to handle an error from this ioctl
> > > gracefully anyway since whether it works or not depends on the gpu
> > > generation and what firmware is loaded right now.
> >
> > We could also define it as
> >
> >         struct drm_nouveau_get_zcull_info {
> >                 __u32 version;
> >                 __u32 _pad;
> >
> >                 union {
> >                         struct drm_nouveau_get_zcull_info_v1 info;
> >                 }
> >         }
> >
> > just to be safe.
> 
> We can do that, although I don't see any other drm drivers using a
> similar pattern anywhere.

I think it's a bit cleaner than adding new members, leave existing ones unset or
add a new IOCTL in the worst case.

Maybe the NVIDIA folks can give us some hint on whether this is expected to
change at some point?