linux-kernel - Re: V4L2 drivers: potentially dangerous and inefficient msecs_to

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Message-ID: <Pine.LNX.4.58.0909141443090.17485@shell2.speakeasy.net>
Date:	Mon, 14 Sep 2009 14:47:22 -0700 (PDT)
From:	Trent Piepho <xyzzy@...akeasy.org>
To:	Andreas Mohr <andi@...as.de>
cc:	Guennadi Liakhovetski <g.liakhovetski@....de>,
	Mauro Carvalho Chehab <mchehab@...radead.org>,
	Luca Risolia <luca.risolia@...dio.unibo.it>,
	linux-media@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: V4L2 drivers: potentially dangerous and inefficient	msecs_to_jiffies()
 calculation

On Mon, 14 Sep 2009, Andreas Mohr wrote:
>                             cam->module_param.frame_timeout *
>                             1000 * msecs_to_jiffies(1) );
> multiple times each.
> What they should do instead is
> frame_timeout * msecs_to_jiffies(1000), I'd think.
> msecs_to_jiffies(1) is quite a bit too boldly assuming
> that all of the msecs_to_jiffies(x) implementation branches
> always round up.

It might also wait far longer than desired.  On a 100 Hz kernel a jiffie is
10 ms.  1000 * msecs_to_jiffies(1) will wait 10 seconds instead of 1.

But, I believe there is an issue with msecs_to_jiffies(x) overflowing for
very high values of x.  I'm not sure where that point is though.
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