Re: [PATCHv2 3/9] v4l: add buffer exporting via dmabuf

[Hans Verkuil <hverkuil@xs4all.nl>]

On Wed August 1 2012 22:49:57 Rémi Denis-Courmont wrote:
> Le mercredi 1 août 2012 14:35:03 Laurent Pinchart, vous avez écrit :
> > > But in general, the V4L element in the pipeline does not know how fast
> > > the downstream element(s) will consume the buffers. Thus it has to copy
> > > from the MMAP buffers into anonymous user memory pending processing.
> > > Then any dequeued buffer can be requeued as soon as possible. In theory,
> > > it might also be that, even though the latency is known, the number of
> > > required buffers exceeds the maximum MMAP buffers count of the V4L
> > > device. Either way, user space ends up doing memory copy from MMAP to
> > > custom buffers.
> > > 
> > > This problem does not exist with USERBUF - the V4L2 element can simply
> > > allocate a new buffer for each dequeued buffer.
> > 
> > What about using the CREATE_BUFS ioctl to add new MMAP buffers at runtime ?
> 
> Does CREATE_BUFS always work while already streaming has already started? If 
> it depends on the driver, it's kinda helpless.

Yes, it does. It's one of the reasons it exists in the first place. But there
are currently only a handful of drivers that implement it. I hope that as
more and more drivers are converted to vb2 that the availability of create_bufs
will increase.

> What's the guaranteed minimum buffer count? It seems in any case, MMAP has a 
> hard limit of 32 buffers (at least videobuf2 has), though one might argue this 
> should be more than enough.

Minimum or maximum? The maximum is 32, that's hardcoded in the V4L2 core.
Although drivers may force a lower maximum if they want. I have no idea
whether there are drivers that do that. There probably are.

The minimum is usually between 1 and 3, depending on hardware limitations.

Regards,

	Hans