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

[Laurent Pinchart <laurent.pinchart@ideasonboard.com>]

Hi Hans,

On Thursday 02 August 2012 09:08:18 Hans Verkuil wrote:
> On Thu August 2 2012 08:56:43 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.
> 
> IF create_bufs is implemented in the driver, THEN you can use it during
> streaming. I.e., it will never return EBUSY as an error due to the fact
> that streaming is in progress.
> 
> Obviously it won't work if the driver didn't implement it in the first
> place.
>
> > > > 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.
> 
> I've no idea. Most will probably abide by the 32 maximum, but without
> analyzing all drivers I can't guarantee it.
> 
> > > 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.
> 
> Just to put your mind at rest: USERPTR mode will *not* disappear or be
> deprecated in any way. It's been there for a long time, it's in heavy use,
> it's easy to use and it will not be turned into a second class citizen,
> because it isn't. Just because there is a new dmabuf mode available doesn't
> mean that everything should be done as a mmap+dmabuf thing.

I disagree with this. Not everything should obviously be done with MMAP + 
DMABUF, but for buffer sharing between devices, we should encourage 
application developers to use DMABUF instead of USERPTR.

-- 
Regards,

Laurent Pinchart