Re: [PATCH v2 25/25] drm/xlnx: Compute dumb-buffer sizes with drm_mode_size_dumb()

[Tomi Valkeinen <tomi.valkeinen@xxxxxxxxxxxxxxxx>]

Hi,

On 16/01/2025 11:38, Laurent Pinchart wrote:
> On Thu, Jan 16, 2025 at 10:43:40AM +0200, Laurent Pinchart wrote:
> > On Wed, Jan 15, 2025 at 02:34:26PM +0000, Daniel Stone wrote:
> > > On Wed, 15 Jan 2025 at 14:20, Tomi Valkeinen wrote:
> > > > No disagreement there, we need CREATE_DUMB2.
> > > >
> > > > My point is that we have the current UAPI, and we have userspace using
> > > > it, but we don't have clear rules what the ioctl does with specific
> > > > parameters, and we don't document how it has to be used.
> > > >
> > > > Perhaps the situation is bad, and all we can really say is that
> > > > CREATE_DUMB only works for use with simple RGB formats, and the behavior
> > > > for all other formats is platform specific. But I think even that would
> > > > be valuable in the UAPI docs.
> > > Yeah, CREATE_DUMB only works for use with simple RGB formats in a
> > > linear layout. Not monochrome or YUV or tiled or displayed rotated or
> > > whatever.
> > >
> > > If it happens to accidentally work for other uses, that's fine, but
> > > it's not generically reliable for anything other than simple linear
> > > RGB. It's intended to let you do splash screens, consoles, recovery
> > > password entries, and software-rendered compositors if you really
> > > want. Anything more than that isn't 'dumb'.
> > We have lots of software out there that rely on CREATE_DUMB supporting
> > YUV linear formats, and lots of drivers (mostly on Arm I suppose) that
> > implement YUV support in CREATE_DUMB. I'm fine replacing it with
> > something better, but I think we need a standard ioctl that can create
> > linear YUV buffers. I've been told many times that DRM doesn't want to
> > standardize buffer allocation further than what CREATE_DUMB is made for.
> > Can we reconsider this rule then ?
> Actually... Instead of adding a CREATE_DUMB2, it would be best on trying
> to leverage DMA heaps and deprecate allocating from the KMS device.
If we allocate from DMA heaps, I think we then need a DRM ioctl which 
will tell you how big buffer(s) you need, based on the size and format.
 Tomi