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

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Date:	Thu, 11 Aug 2011 11:49:55 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
cc:	Mauro Carvalho Chehab <mchehab@...radead.org>,
	Hans de Goede <hdegoede@...hat.com>,
	Theodore Kilgore <kilgota@...ach.math.auburn.edu>,
	Sarah Sharp <sarah.a.sharp@...ux.intel.com>,
	Greg KH <greg@...ah.com>, <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 Thu, 11 Aug 2011, Alan Cox wrote:

> Actually there are more issues than that - you've also got to worry about
> a security/permission model, and that is hard to get right, especially if
> you are not very careful that anything that can be retrieved which might
> violate the security model (eg the last frame on the capture) has been
> blanked before handover etc.

As far as I can tell, these same security issues exist today.  I don't 
see them getting any worse than they are now.

> And applications that are touching both video (even indirectly) and still
> camera may get surprise deadlocks if they accidentally reference both the
> still and video device even via some library or service.

No, not deadlocks.  Just -EBUSY errors.

> > > Well, a user program can assume that the kernel driver left the device
> > > in a clean state.  The reverse isn't always true, however -- it's one
> 
> Not it cannot - the user program doesn't know
> 
> a) if the kernel driver has ever been loaded
> b) the values the kernel driver leaves set (and those will change no
> doubt at times)
> c) if the camera has been plugged and unplugged and not yet had the
> kernel driver loaded

That's true.  The program can't assume that a kernel driver was ever 
bound to the device; all it can assume is that _if_ a kernel driver was 
bound then it left the device in a sane state -- whatever "sane" might 
mean in this context.

> To me it sounds like a recipe for disaster. For those tiny number of
> devices involved just use V4L and if need be some small V4L tweaks to
> handle still mode. In most cases the interface is basically identical and
> I'd bet much of the code is identical too.

I'm not against moving the whole thing into the kernel.  I'm just
pointing out that an easier-to-code-up solution will accomplish much
the same result.

Alan Stern

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