[PATCH] drm: document expectations for GETFB2 handles

[Daniel Stone]

Hi,

On Thu, 16 Feb 2023 at 09:25, Simon Ser <contact at emersion.fr> wrote:
> On Thursday, February 16th, 2023 at 10:11, Pekka Paalanen <ppaalanen at gmail.com> wrote:
> > Btw. does this also mean that if you use GETFB2 to get handle A, you
> > export that as dmabuf and import in the same open device instance, you
> > again get handle A?
>
> I haven't tested it, but I believe that is correct.
>
> > IOW, you should never ever export a dmabuf of what you got with
> > GETFB2. If one did, one might import it oneself via GBM, breaking all
> > reference counting. But you also cannot "just leak" the handle A,
> > because if GBM happens to run on a different DRM device opened
> > instance, GBM would get a different handle to own.
> >
> > That's... err. How is a compositor supposed to do transition animation
> > from an old FB to its own thing? I guess mmap + glTexImage2D equivalent
> > to make a copy of the old FB so one can use it as a texture?
>
> I think the compositor can export the handle as DMA-BUF, then close the
> handle immediately. Then go about its business as usual.

Yeah, I think either of those two are the most sensible. We did go
back and forth over the semantics a few times - part over mail and
part over IRC - and the eventual conclusion was to match GetFB to make
it easier for users to transition, but to de-duplicate the handles
_within_ the call for consistency with the rest of everything else.
It's not the most pleasant compromise, but eh.

Cheers,
Daniel