[PATCH] media: videobuf2-core.c: check if all buffers have the same number of planes

[Sakari Ailus]

Hi Hans, Laurent,

On Wed, Aug 16, 2023 at 05:34:32PM +0300, Laurent Pinchart wrote:
> Hi Hans,
> 
> Thank you for the patch.
> 
> On Wed, Aug 16, 2023 at 02:47:33PM +0200, Hans Verkuil wrote:
> > Currently if VIDIOC_CREATE_BUFS is called to add new buffers, then the requested
> > number of planes per buffer might be different from the already allocated buffers.
> > 
> > However, this does not make a lot of sense and there are no drivers that allow
> > for variable number of planes in the allocated buffers.
> > 
> > While it is possible do this today, when such a buffer is queued it will fail
> > in the buf_prepare() callback where the plane sizes are checked if those are
> > large enough. If fewer planes were allocated, then the size for the missing
> > planes are 0 and the check will return -EINVAL.
> > 
> > But it is much better to do this check in VIDIOC_CREATE_BUFS.
> 
> I don't think this is a good idea. One important use case for
> VIDIOC_CREATE_BUFS is to allocate buffers for a different format than
> the one currently configured for the device, to prepare for a future
> capture (or output) session with a different configuration. This patch
> would prevent that.

I'd prefer to keep this capability in videobuf2, too. Although... one way
achieve that could be to add a flag (or an integer field) in struct
vb2_queue to tell vb2 core that the driver wants to do the num_planes
checks by itself.

It'd be also nicer, considering the end result, to configure this when
setting up the queue, rather than based on the first buffer created. This
would involve changing a large number of drivers though.

-- 
Regards,

Sakari Ailus