Re: [RFC PATCH 0/8] V4L BKL removal: first round

[Arnd Bergmann <arnd@arndb.de>]

On Tuesday 16 November 2010, Hans Verkuil wrote:
> > I think there is a misunderstanding. One V4L device (e.g. a TV capture
> > card, a webcam, etc.) has one v4l2_device struct. But it can have multiple
> > V4L device nodes (/dev/video0, /dev/radio0, etc.), each represented by a
> > struct video_device (and I really hope I can rename that to v4l2_devnode
> > soon since that's a very confusing name).
> >
> > You typically need to serialize between all the device nodes belonging to
> > the same video hardware. A mutex in struct video_device doesn't do that,
> > that just serializes access to that single device node. But a mutex in
> > v4l2_device is at the right level.

Ok, got it now.

> A quick follow-up as I saw I didn't fully answer your question: to my
> knowledge there are no per-driver data structures that need a BKL for
> protection. It's definitely not something I am worried about.

Good. Are you preparing a patch for a per-v4l2_device then? This sounds
like the right place with your explanation. I would not put in the
CONFIG_BKL switch, because I tried that for two other subsystems and got
called back, but I'm not going to stop you.

As for the fallback to a global mutex, I guess you can set the
videodev->lock pointer and use unlocked_ioctl for those drivers
that do not use a v4l2_device yet, if there are only a handful of them.

	Arnd