linux-kernel - Re: Question: how to copy to user space from a tasklet

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Message-Id: <201010151059.40757.arnd@arndb.de>
Date:	Fri, 15 Oct 2010 10:59:40 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	Stefan Richter <stefanr@...6.in-berlin.de>
Cc:	Tom Brown <sa212+emis@...onix.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Yong Zhang <yong.zhang@...driver.com>
Subject: Re: Question: how to copy to user space from a tasklet

On Friday 15 October 2010 07:57:59 Stefan Richter wrote:
> Tom Brown wrote:
> > [Sorry to have to ask this here - I can't find any driver-specific
> > newsgroup or mailing lists - are there any??]
> 
> linux-newbie perhaps.
> 
> > I have a driver which needs to copy device data direct to user space
> > from a tasklet.
> 
> One way would be to use mmap() to allocate the DMA buffer and insert it into
> the user address space before DMA commences.  An additional poll/read/write
> based protocol or so can be used for buffer management during DMA.

Agreed, that would be best, it's a lot easier than the way Tom describes
the driver to work today.

Just for completeness: doing DMA to random user address would require the
process to register the area using some home grown interface, so that the
driver can do get_user_pages/kmap on all of them and subsequently pass
around an array of pages, while keeping track of ulimits and coherency
with the existing kernel mappings. Don't do that.

	Arnd
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