linux-kernel - Re: [PATCH resent v2 1/2] ioport : add DEFINE_RES_DMA_SIZE

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Date:	Mon, 6 Feb 2012 00:22:13 +0800
From:	Huang Shijie <shijie8@...il.com>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	Huang Shijie <b32955@...escale.com>, akpm@...ux-foundation.org,
	shawn.guo@...aro.org, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org,
	u.kleine-koenig@...gutronix.de
Subject: Re: [PATCH resent v2 1/2] ioport : add DEFINE_RES_DMA_SIZE_NAMED macro

On Sun, Feb 5, 2012 at 8:34 PM, Russell King - ARM Linux
<linux@....linux.org.uk> wrote:
> On Sun, Feb 05, 2012 at 07:53:31PM +0800, Huang Shijie wrote:
>> Does any one have any comments about this patch?
>
> Yes.  Having a flat numberspace for DMAs invites the same problems that
> we have with the flat IRQ numberspace when it comes to having multiple
> DMA controllers.
>
> With DMA engine drivers, channels are allocated using a match function
> and unspecified match data.  While a linear numberspace can be used,
> that sets itself up for clashing between different DMA controllers
> especially if the numberspace overlaps.
>
> What's missing from this patch set is an illustration of how this will
> be used.
>
> Another issue here is that having the peripheral drivers know about the
> exact match function and what data the DMA engines require ties the
> periperal drivers to their respective DMA engine.  One of the points of
> DMA engine is to separate that knowledge, so that the same peripheral IP
> can be re-used with different DMA engines.
>
> To encode knowledge of the DMA engine into the peripheral driver basically
> destroys the purpose of having a DMA engine API.
>
> So, I believe that _if_ we are going down the path of using the DMA engine
> API for our SoCs, we really don't want to invent a linear DMA numberspace
> by putting DMA numbers into resources.

ok. thanks.


Huang Shijie
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