linux-kernel - Re: Should I use kmap or kmap_atomic to map user pages that will be written in a loop ?

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Message-ID: <20090726152655.7488abe4@bike.lwn.net>
Date:	Sun, 26 Jul 2009 15:26:55 -0600
From:	Jonathan Corbet <corbet@....net>
To:	Laurent Pinchart <laurent.pinchart@...net.be>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: Should I use kmap or kmap_atomic to map user pages that will be
 written in a loop ?

On Sat, 25 Jul 2009 23:41:47 +0200
Laurent Pinchart <laurent.pinchart@...net.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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