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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Date:	Wed, 29 Jul 2009 10:58:23 +0200
From:	Laurent Pinchart <laurent.pinchart@...net.be>
To:	Jonathan Corbet <corbet@....net>
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 ?

Hi Jonathan,

On Monday 27 July 2009 23:11:58 Jonathan Corbet wrote:
> On Sun, 26 Jul 2009 23:52:09 +0200 Laurent Pinchart 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.

Thanks. I'll try that, it will make my life easier.

Regards,

Laurent Pinchart

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