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

["Rémi Denis-Courmont" <remi@remlab.net>]

Le jeudi 2 août 2012 09:35:58 Hans Verkuil, vous avez écrit :
> On Wed August 1 2012 22:49:57 Rémi Denis-Courmont wrote:
> > > 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.

That's contradictory. If most drivers do not support it, then it won't work 
during streaming.

> > 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 smallest of the maxima of all drivers.

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

And that's clearly insufficient without memory copy to userspace buffers.

It does not seem to me that CREATE_BUFS+MMAP is a useful replacement for 
REQBUFS+USERBUF then.

-- 
Rémi Denis-Courmont
http://www.remlab.net/
http://fi.linkedin.com/in/remidenis