linux-kernel - Re: [RFC/PARTIAL PATCH 0/3] dma: passing "attributes" to dma_map

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Message-ID: <20080109210542.GP23661@sgi.com>
Date:	Wed, 9 Jan 2008 13:05:42 -0800
From:	akepner@....com
To:	Roland Dreier <rdreier@...co.com>
Cc:	James Bottomley <James.Bottomley@...senPartnership.com>,
	Tony Luck <tony.luck@...el.com>,
	Grant Grundler <grundler@...isc-linux.org>,
	Jesse Barnes <jbarnes@...tuousgeek.org>,
	Jes Sorensen <jes@....com>,
	Randy Dunlap <randy.dunlap@...cle.com>,
	David Miller <davem@...emloft.net>,
	Muli Ben-Yehuda <muli@...ibm.com>, linux-kernel@...r.kernel.org
Subject: Re: [RFC/PARTIAL PATCH 0/3] dma: passing "attributes" to dma_map_* routines

On Wed, Jan 09, 2008 at 01:00:38PM -0800, Roland Dreier wrote:
> .....
> The reason this hasn't been an issue until now is that almost all
> drivers work correctly if the Altix code just sets the "flush" bit for
> memory allocated via the consistent/coherent allocators.  However, if
> we want the device to write to userspace memory, this doesn't work
> (and mapping coherent memory allocated in the kernel into userspace is
> a mess on other platforms, because it unnecessarily consumes lowmem
> and/or kernel address space).
> 

And the only way that user level CQs work on Altix now is 
that we apply our own patches to allocate them in the 
kernel (with dma_alloc_coherent()) them mmap() them back 
to user space. A bug in one of these patches recently led  
to considerable drama with several customers - I'd love 
to get a fix in mainline so we can drop our patches and 
avoid the possibility such theatrics in the future ;-)

-- 
Arthur

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