linux-kernel - Re: [RFC v2 4/5] dmaengine: Driver for the Synopsys DesignWare DMA controller

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Date:	Wed, 30 Jan 2008 11:53:24 -0700
From:	"Dan Williams" <dan.j.williams@...el.com>
To:	"Haavard Skinnemoen" <hskinnemoen@...el.com>
Cc:	linux-kernel@...r.kernel.org,
	"Shannon Nelson" <shannon.nelson@...el.com>,
	"David Brownell" <david-b@...bell.net>, kernel@...32linux.org,
	"Francis Moreau" <francis.moro@...il.com>,
	"Paul Mundt" <lethal@...ux-sh.org>,
	"Vladimir A. Barinov" <vbarinov@...mvista.com>,
	"Pierre Ossman" <drzeus-list@...eus.cx>
Subject: Re: [RFC v2 4/5] dmaengine: Driver for the Synopsys DesignWare DMA controller

On Jan 29, 2008 11:10 AM, Haavard Skinnemoen <hskinnemoen@...el.com> wrote:
[..]
> The dmatest client shows no problems, but the performance is not as
> good as it should be yet -- iperf shows a slight slowdown when
> enabling TCP receive copy offload. This is probably because the
> controller is set up to always do byte transfers; I'll try to optimize
> this, but if someone can tell me if there any guaranteed alignment
> requirements for the users of the DMA engine API, that would help a
> lot.
>

dmaengine punts to the dma-mapping api.  So no, there are no alignment
guarantees.  The performance loss is probably more related to the
cache synchronization overkill of get_user_pages().  I/O incoherent
architectures end up synchronizing entire pages when we only need to
sync a kilobyte or two in this path.

--
Dan
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