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

[Laurent Pinchart]

Hi Rémi,

On Thursday 02 August 2012 09:56:43 Rémi Denis-Courmont wrote:
> 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.

That's the minimum number of buffers *required* by the hardware. You can add 
up to 32 buffers, I'm not aware of any driver that would prevent that.

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

-- 
Regards,

Laurent Pinchart