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

["Danilo Krummrich" <dakr@kernel.org>]

On Thu Feb 5, 2026 at 2:16 AM CET, Dave Airlie wrote:
> On Fri, 28 Mar 2025 at 21:09, Danilo Krummrich <dakr@kernel.org> wrote:
>>
>> (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?
>
> I think it's an ioctl, let's just leave out version/pad, adding a new
> ioctl isn't a major trouble if the world decides we need to do it.
>
> Also you can extend ioctls with new fields at the end without
> problems, so if it's just extra info it'll likely be hw specific and
> be part of enabling a new GPU.

Yeah, that's why I said "it's a bit cleaner than adding new members, leave
existing ones unset or add a new IOCTL in the worst case" above. :)

Anyways, this is quite a while ago and things have changed. I think we can
afford to be a bit less future proof in terms of new GPU (and firmware) support.
:)