Re: [PATCH 1/1] V4L: stk11xx, add a new webcam driver

[Thierry Merle <thierry.merle@free.fr>]

Mauro Carvalho Chehab a écrit :
> Em Ter, 2007-05-29 às 21:04 +0200, Thierry Merle escreveu:
>   
> > Mauro Carvalho Chehab a écrit :
> >     
> > >   
> > >   
> > >       
> > > > > Hi Mauro and Markus,
> > > > > Just to summ up what I understood we need:
> > > > >
> > > > > What do we need in userspace, only for v4l (dvb is not concerned):
> > > > > - colorspace translations
> > > > > - filters that be done in hardware if the selected hardware can, 
> > > > > otherwise software plugin
> > > > > - decompression algorithm like stk11xx or usbvision (the decompression 
> > > > > algorithm is in kernelspace since it is of linear complexity but shall 
> > > > > be moved to userspace)
> > > > >       
> > > > >           
> > > Yes. The first focus, IMO, should be the last one.
> > >
> > >   
> > >       
> > OK, so we must open the V4L2 API to add a custom pixel format.
> >     
> I don't think we need to change the API. The API already exports the
> format, as a fourcc-like info. We just need to add newer codes as
> they'll be added into kernel.
>
>   
> > While brainstorming, I thought it would be funny to create userspace 
> > drivers like FUSE based projects do.
> > Applications would see nothing but a new device driver to access, all 
> > decompression algorithm would be in userspace.
> > The path from the application to the v4l2 kernel driver:
> > [Application]<->[/mnt/video0 (created by FUSE-like userspace 
> > lib)]<->[/dev/video0 (created by kernel v4l2 driver)]
> > There are limitations that I don't know, but this would be transparent 
> > for the applications.
> >     
>
> This seems to be an interesting approach.
>
>   
Interesting but impossible to do for ioctl calls.
When the application does a ioctl(fd_of_mnt_video0,VIDIOC_G_FMT,&arg) 
for example, there is no way for the userspace helper to catch this ioctl.
The application could only open/read from the userspace helper's file 
/mnt/video0.
ioctl would still have to be done on the kernel device driver.
I thought also about a /proc interface for decompression algorithms (a 
helper would listen on a /proc file and write on another /proc file) but 
/proc is not designed for that kind of thing.
A separate library seems to be the simplest solution.
> > > Yes, it is possible. In fact, some devices currently work by generating
> > > a common audio and video stream. The driver may just send the packet
> > > as-is, leaving to the userspace API the function to de-merge and
> > > synchronize audio and video.
> > >   
> > >       
> > So we have a userspace lib that will demux/uncompress device driver output.
> > This lib will know each device driver particularities to know how to 
> > demux/uncompress in a a/v format that will be understood by the application.
> >     
>
> Yes. However, we should try to do loose coupling whenever possible, to
> keep maintainership of the "library"(*) as simple as possible.
>
> (*) With this approach, it seems that it will turn into a helper daemon,
> instead of just a library.
>
> Cheers,
> Mauro
>
>   
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