linux-kernel - Re: USB mini-summit at LinuxCon Vancouver

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Message-ID: <Pine.LNX.4.44L0.1108101156350.1917-100000@iolanthe.rowland.org>
Date:	Wed, 10 Aug 2011 12:09:11 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Theodore Kilgore <kilgota@...ach.math.auburn.edu>
cc:	Hans de Goede <hdegoede@...hat.com>,
	Sarah Sharp <sarah.a.sharp@...ux.intel.com>,
	Greg KH <greg@...ah.com>,
	Mauro Carvalho Chehab <mchehab@...radead.org>,
	<linux-usb@...r.kernel.org>, <linux-media@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>,
	<libusb-devel@...ts.sourceforge.net>,
	Alexander Graf <agraf@...e.de>,
	Gerd Hoffmann <kraxel@...hat.com>, <hector@...cansoft.com>,
	Jan Kiszka <jan.kiszka@...mens.com>,
	Stefan Hajnoczi <stefanha@...ux.vnet.ibm.com>,
	<pbonzini@...hat.com>, Anthony Liguori <aliguori@...ibm.com>,
	Jes Sorensen <Jes.Sorensen@...hat.com>,
	Oliver Neukum <oliver@...kum.org>, Felipe Balbi <balbi@...com>,
	Clemens Ladisch <clemens@...isch.de>,
	Jaroslav Kysela <perex@...ex.cz>, Takashi Iwai <tiwai@...e.de>,
	Laurent Pinchart <laurent.pinchart@...asonboard.com>,
	Adam Baker <linux@...er-net.org.uk>
Subject: Re: USB mini-summit at LinuxCon Vancouver

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

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