linux-kernel - Re: [PATCH 0/5] Pushdown bkl from v4l ioctls

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Message-Id: <201004290938.09573.arnd@arndb.de>
Date:	Thu, 29 Apr 2010 09:38:09 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	Laurent Pinchart <laurent.pinchart@...asonboard.com>
Cc:	Hans Verkuil <hverkuil@...all.nl>,
	Frederic Weisbecker <fweisbec@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	John Kacur <jkacur@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Jan Blunck <jblunck@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Mauro Carvalho Chehab <mchehab@...radead.org>,
	Greg KH <gregkh@...e.de>,
	Linux Media Mailing List <linux-media@...r.kernel.org>
Subject: Re: [PATCH 0/5] Pushdown bkl from v4l ioctls

On Thursday 29 April 2010 09:10:42 Laurent Pinchart wrote:
> On Thursday 29 April 2010 08:44:29 Hans Verkuil wrote:
> >
> > 3) Investigate what needs to be done to replace the bkl with a v4l2-dev.c
> > global mutex. Those drivers that call the bkl themselves should probably be
> > converted to do proper locking, but there are only about 14 drivers that do
> > this. The other 60 or so drivers should work fine if a v4l2-dev global lock
> > is used. At this point the bkl is effectively removed from the v4l
> > subsystem.
> > 
> > 4) Work on the remaining 60 drivers to do proper locking and get rid of the
> > v4l2-dev global lock. This is probably less work than it sounds.
> > 
> > Since your patch moves everything down to the driver level it will actually
> > make this work harder rather than easier. And it touches almost all drivers
> > as well.
> 
> Every driver will need to be carefully checked to make sure the BKL can be 
> replaced by a v4l2-dev global mutex. Why would it be more difficult to do so 
> if the BKL is pushed down to the drivers ?

Note that you can completely skip the step of a v4l2-dev global mutex with
Frederic's patch. This is the only use of the BKL in the common v4l2
code as far as I can tell, so instead of introducing yet another global
lock, you can go straight to stage 4 and look at each driver separately,
possibly introducing a per driver lock.

	Arnd
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