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

[John Hubbard <jhubbard@nvidia.com>]

On 2/4/26 5:16 PM, 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?

"Predictions are hard, especially about the future." :)

> 
> 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.
> 

That seems reasonable to me.

thanks,
-- 
John Hubbard