From: Laurent Pinchart <laurent.pinchart@skynet.be>
To: Jonathan Corbet <corbet@lwn.net>
Subject: Re: Should I use kmap or kmap_atomic to map user pages that will be written in a loop ?
Date: Sun, 26 Jul 2009 23:52:09 +0200
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Hi Jonathan,

first of all thanks for your answer.

On Sunday 26 July 2009 23:26:55 Jonathan Corbet wrote:
> 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.

Ok thanks.

> 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.

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 ?

Regards,

Laurent Pinchart

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