linux-kernel - Re: [PATCH] dma: add new DMA control commands

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Date:	Thu, 18 Oct 2012 09:52:08 +0100
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Huang Shijie <b32955@...escale.com>
Cc:	Vinod Koul <vinod.koul@...ux.intel.com>, djbw@...com,
	khali@...ux-fr.org, ben-linux@...ff.org, w.sang@...gutronix.de,
	cjb@...top.org, dwmw2@...radead.org, lrg@...com,
	broonie@...nsource.wolfsonmicro.com, perex@...ex.cz, tiwai@...e.de,
	shawn.guo@...aro.org, marex@...x.de,
	artem.bityutskiy@...ux.intel.com, linux-kernel@...r.kernel.org,
	linux-i2c@...r.kernel.org, linux-mmc@...r.kernel.org,
	linux-mtd@...ts.infradead.org, alsa-devel@...a-project.org,
	linux-arm-kernel@...ts.infradead.org,
	Huang Shijie <shijie8@...il.com>
Subject: Re: [PATCH] dma: add new DMA control commands

On Thu, Oct 18, 2012 at 02:45:41PM +0800, Huang Shijie wrote:
> 于 2012年10月18日 14:18, Vinod Koul 写道:
>> Why cant you do start (prepare clock etc) when you submit the descriptor
>> to dmaengine. Can be done in tx_submit callback.
>> Similarly remove the clock when dma transaction gets completed.
> I ever thought this method too.
>
> But it will become low efficient in the following case:
>
>   Assuming the gpmi-nand driver has to read out 1024 pages in one  
> _SINGLE_ read operation.
> The gpmi-nand will submit the descriptor to dmaengine per page. So with  
> your method,
> the system will repeat the enable/disable dma clock 1024 time. At every  
> enable/disable dma clock,
> the system has to enable the clock chain and it's parents ...

And what if you stop using clk_prepare_enable(), and prepare the clock
when the channel is requested and only use clk_enable() in the tx_submit
method?
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