Date: Sun, 26 Jul 2009 15:26:55 -0600
From: Jonathan Corbet <corbet@lwn.net>
To: Laurent Pinchart <laurent.pinchart@skynet.be>
Cc: linux-kernel@vger.kernel.org
Subject: Re: Should I use kmap or kmap_atomic to map user pages that will be
 written in a loop ?
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In-Reply-To: <200907252341.48526.laurent.pinchart@skynet.be>
References: <200907252341.48526.laurent.pinchart@skynet.be>
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On Sat, 25 Jul 2009 23:41:47 +0200
Laurent Pinchart <laurent.pinchart@skynet.be> wrote:

> Pages will be written to from the kernel in USB interrupt context. I can then 
> either kmap_atomic() pages before copying data and kunmap_atomic() them right 
> after, or kmap() them once at the beginning of the video stream and keep them 
> mapped until the end.

Video buffers can be big, and the streaming interface requires at least
two of them.  That's a lot of kmap'd pages.  It seems to me that
kmap_atomic() is the way to go for something like this.

But, then, these are user-space buffers, and you're seemingly buffering
the data through kernel space buffers first?  It seems like using
copy_to_user() in a workqueue (or a threaded interrupt handler) might be
a more straightforward way to go, unless I'm missing something.

jon
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