linux-kernel - Re: [PATCH] fsldma: add support to 36-bit physical address

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Date:	Tue, 21 Sep 2010 16:34:56 -0500
From:	Scott Wood <scottwood@...escale.com>
To:	Timur Tabi <timur.tabi@...il.com>
CC:	Kumar Gala <galak@...nel.crashing.org>, dan.j.williams@...el.com,
	linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fsldma: add support to 36-bit physical address

On Tue, 21 Sep 2010 16:24:10 -0500
Timur Tabi <timur.tabi@...il.com> wrote:

> On Tue, Sep 21, 2010 at 7:55 AM, Kumar Gala <galak@...nel.crashing.org> wrote:
> >
> > On Sep 21, 2010, at 5:57 AM, Li Yang wrote:
> >
> >> Signed-off-by: Li Yang <leoli@...escale.com>
> >> ---
> >
> > We really should have a sentence about how or why this works to address 36-bit addressing.
> 
> For example, I would like to know which memory is going to be
> allocated above 4GB.  I don't know much about the kernel's async
> library, but my understanding is that fsldma does not allocate any of
> the memory buffers that it copies data to/from.

This doesn't control allocation (it probably should with
dma_alloc_coherent, though I don't see it in the code), it controls
whether swiotlb will create a bounce buffer -- defeating the point of
using DMA to accelerate a memcpy.

-Scott

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