linux-kernel - Re: How to avoid data copies in a driver ?

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Message-ID: <482D416B.8060501@goop.org>
Date:	Fri, 16 May 2008 09:10:19 +0100
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Francis Moreau <francis.moro@...il.com>
CC:	"linux-os (Dick Johnson)" <linux-os@...logic.com>,
	linux-kernel@...r.kernel.org
Subject: Re: How to avoid data copies in a driver ?

Francis Moreau wrote:
> I'm suprised because what I need doens't seem so uncommon, usually
> devices send or
> receive data to/from files. So a helper (system call ?) to achieve
> that other than the basic
> read/write seems needed, no ?
>   

It's fairly rare to have an application which requires moving data to 
file with absolutely no processing; normally there's at least a bit of 
massaging/parsing/etc.  If that's really what you want to do, maybe you 
can do it with splice?  I haven't looked at it at all, but the intention 
is that you can splice file descriptors together, so you can splice your 
device fd to a file fd and have it all just work...

Alternatively you could read() from your device into a mmaped file.  
That's a single copy from device to file, which is about the best you 
can do without going to heroic lengths.

Also, it really depends on your application.  Is it a high-bandwidth 
thing in which the copy is a huge cost?  Or do you want to eliminate the 
copies because it seems like a nice thing to do?

    J
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