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: <20090727151158.2c5db4eb@bike.lwn.net>
Date:	Mon, 27 Jul 2009 15:11:58 -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 Sun, 26 Jul 2009 23:52:09 +0200
Laurent Pinchart <laurent.pinchart@...net.be> wrote:

> I receive data from the USB subsystem in URB buffers, which are small kernel 
> buffers. As I have to strip headers from those buffers, I can't initialize the 
> URBs to copy data directly to the userspace buffers, so there's at least one 
> memcpy operation involved :-S
> 
> I could indeed append the URBs to a list in the callback called from interrupt 
> context, and process them from a threaded interrupt handler. Would it make 
> much difference ?

Moving the actual copying out of interrupt context seems like a good
idea.  And if you can get away from the whole get_user_pages() and
kmap() mess into a straight copy-to-user-space situation, you'll make
your life easier.  So that would be my recommendation, yes.

jon
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