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

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Message-ID: <20100921164944.164ed0af@udp111988uds.am.freescale.net>
Date:	Tue, 21 Sep 2010 16:49:44 -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 16:43:12 -0500
Timur Tabi <timur.tabi@...il.com> wrote:

> On Tue, Sep 21, 2010 at 4:34 PM, Scott Wood <scottwood@...escale.com> wrote:
> 
> > 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.
> 
> But it would do that only for the 'dev' used in the dma_set_mask()
> call.  That dev is only used here:
> 
> 	chan->desc_pool = dma_pool_create("fsl_dma_engine_desc_pool",
> 					  chan->dev,
> 					  sizeof(struct fsl_desc_sw),
> 					  __alignof__(struct fsl_desc_sw), 0);
> 
> Since we don't DMA the descriptors themselves, I just don't see how
> this patch does anything.

Look in dmaengine.c, there are calls to dma_map_single() and
dma_map_page(), using what I assume is that same device pointer --
unless there's confusion between the channel and the controller.

-Scott

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