Re: [PATCH v9 11/15] vb2: add in-fence support to QBUF

[Brian Starkey]

Hi,

On Tue, May 08, 2018 at 08:18:06PM -0300, Gustavo Padovan wrote:
> Hi Hans,
>
> On Mon, 2018-05-07 at 14:07 +0200, Hans Verkuil wrote:
> > On 04/05/18 22:06, Ezequiel Garcia wrote:
> > > From: Gustavo Padovan <gustavo.padovan@xxxxxxxxxxxxx>

[snip]

> > > diff --git a/include/media/videobuf2-core.h
> > > b/include/media/videobuf2-core.h
> > > index 364e4cb41b10..28ce8f66882e 100644
> > > --- a/include/media/videobuf2-core.h
> > > +++ b/include/media/videobuf2-core.h
> > > @@ -17,6 +17,7 @@
> > >  #include <linux/poll.h>
> > >  #include <linux/dma-buf.h>
> > >  #include <linux/bitops.h>
> > > +#include <linux/dma-fence.h>
> > >
> > >  #define VB2_MAX_FRAME	(32)
> > >  #define VB2_MAX_PLANES	(8)
> > > @@ -255,12 +256,21 @@ struct vb2_buffer {
> > >  	 * done_entry:		entry on the list that
> > > stores all buffers ready
> > >  	 *			to be dequeued to userspace
> > >  	 * vb2_plane:		per-plane information; do not
> > > change
> > > +	 * in_fence:		fence received from vb2 client
> > > to wait on before
> > > +	 *			using the buffer (queueing to
> > > the driver)
> > > +	 * fence_cb:		fence callback information
> > > +	 * fence_cb_lock:	protect callback signal/remove
> > >  	 */
> > >  	enum vb2_buffer_state	state;
> > >
> > >  	struct vb2_plane	planes[VB2_MAX_PLANES];
> > >  	struct list_head	queued_entry;
> > >  	struct list_head	done_entry;
> > > +
> > > +	struct dma_fence	*in_fence;
> > > +	struct dma_fence_cb	fence_cb;
> > > +	spinlock_t              fence_cb_lock;
> > > +
> >
> > So for the _MPLANE formats this is one fence for all planes. Which
> > makes sense, but how
> > does drm handle that? Also one fence for all planes?
>
> Yes, this is one fence for all planes.
>
> The DRM concept for planes is a totally different concept and is
> basically a representation of an user definable square on the screen,
> and to that plane there in one framebuffer attached - display hw has no
> such a multiplanar for the same image AFAICT. So you probably need some
> blit to convert the v4l2 multiplanar to a DRM framebuffer.

Lots of display hardware can do multi-planar formats, and there's
space in struct drm_framebuffer for up to 4 buffer handles (e.g. 3
handles are passed for Luma, Cr, and Cb when the framebuffer format is
DRM_FORMAT_YUV420) - like V4L2 MPLANE.

The V4L2 code here matches with the DRM "explicit sync"
(IN_FENCE_FD/OUT_FENCE_PTR) stuff, which is probably what we want.
The main difference is that in DRM, explicit fences aren't associated
with framebuffers, they're associated with the things using the
framebuffers - but practically it doesn't make a difference.

There can be per-buffer "implicit sync" via dma-buf reservation
objects, but I don't think this series should attempt to deal with
that.

Cheers,
-Brian

> > I think there should be a comment about this somewhere.
>
> Yes, we've been over this exact discussion a few times :)
> Having entirely different things with the same name is quite confusing.
>
> Regards,
>
> Gustavo
>
> --
> Gustavo Padovan
> Principal Software Engineer
> Collabora Ltd.