linux-kernel - Re: [PATCH] fsldma: use PCI Read Multiple command

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Message-ID: <2a27d3730904270316u6c06eafeja75f6e9e5c83ed29@mail.gmail.com>
Date:	Mon, 27 Apr 2009 18:16:19 +0800
From:	Li Yang <leoli@...escale.com>
To:	Liu Dave-R63238 <DaveLiu@...escale.com>
Cc:	Ira Snyder <iws@...o.caltech.edu>, linux-kernel@...r.kernel.org,
	linuxppc-dev@...abs.org, Dan Williams <dan.j.williams@...el.com>
Subject: Re: [PATCH] fsldma: use PCI Read Multiple command

On Mon, Apr 27, 2009 at 5:09 PM, Liu Dave-R63238 <DaveLiu@...escale.com> wrote:
>> By default, the Freescale 83xx DMA controller uses the PCI Read Line
>> command when reading data over the PCI bus. Setting the
>> controller to use the PCI Read Multiple command instead allows the
>> controller to read much larger bursts of data, which provides a
> drastic
>> speed increase.
>
> IIRC, the default for 83xx DMA controller uses the PCI mem read
> command, not mem read line.
>
> You are assuming the PCI memory space is prefetchable( no side effect)
> for DMA.
> Is it possible that DMA is from non-prefetchable memory space?

I guess it's not reasonable to use DMA from non-prefetchable memory.
So it's up to the driver which uses the DMA engine to prevent from
using DMA API on non-prefetchable memory.

- Leo
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