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Date: Wed, 10 Aug 2011 12:09:11 -0400 (EDT)
From: Alan Stern <stern@rowland.harvard.edu>
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To: Theodore Kilgore <kilgota@banach.math.auburn.edu>
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Subject: Re: USB mini-summit at LinuxCon Vancouver
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On Wed, 10 Aug 2011, Theodore Kilgore wrote:

> > Okay, I didn't realize that the different cameras used different webcam 
> > drivers as well as different stillcam drivers.
> 
> Oh, yes. They are Proprietary devices. And that means what it says. :-)
> And all different from each other, too.
>  
> > As far as I can see, there's nothing to stop anybody from adding the 
> > stillcam functionality into the webcam drivers right now.  If some 
> > common code can be abstracted out into a shared source file, so much 
> > the better.
> > 
> > That would solve the problem, right?
> 
> I think everyone involved believes that it would solve the problem. 
> 
> The question has been all along whether or not there is any other way 
> which would work. Also the question of what, exactly, "belongs" in the 
> kernel and what does not. For, if something has been historically 
> supported in userspace (stillcam support, in this case) and has worked 
> well there, I would think it is kind of too bad to have to move said 
> support into the kernel just because the same hardware requires kernel 
> support for another functionality and the two sides clash. I mean, the 
> kernel is already big enough, no? But the logic that Hans has set forth 
> seems rather compelling. 

The alternative seems to be to define a device-sharing protocol for USB
drivers.  Kernel drivers would implement a new callback (asking them to
give up control of the device), and usbfs would implement new ioctls by
which a program could ask for and relinquish control of a device.  The
amount of rewriting needed would be relatively small.

A few loose ends would remain, such as how to handle suspends, resumes,
resets, and disconnects.  Assuming usbfs is the only driver that will
want to share a device in this way, we could handle them.

Hans, what do you think?

Alan Stern