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

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Message-ID: <CAJe_ZheWd3scXDhWukeZMuOOz8+bZRwZOw_ftDW8wmkqq9H45Q@mail.gmail.com>
Date:	Thu, 18 Oct 2012 20:55:30 +0530
From:	Jassi Brar <jaswinder.singh@...aro.org>
To:	Huang Shijie <shijie8@...il.com>
Cc:	Huang Shijie <b32955@...escale.com>,
	Vinod Koul <vinod.koul@...ux.intel.com>, marex@...x.de,
	alsa-devel@...a-project.org, shawn.guo@...aro.org,
	linux@....linux.org.uk, tiwai@...e.de,
	artem.bityutskiy@...ux.intel.com,
	broonie@...nsource.wolfsonmicro.com, linux-mmc@...r.kernel.org,
	w.sang@...gutronix.de, perex@...ex.cz,
	linux-mtd@...ts.infradead.org, linux-i2c@...r.kernel.org,
	ben-linux@...ff.org, djbw@...com, khali@...ux-fr.org,
	cjb@...top.org, dwmw2@...radead.org, lrg@...com,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH] dma: add new DMA control commands

On 18 October 2012 20:48, Huang Shijie <shijie8@...il.com> wrote:
> On Thu, Oct 18, 2012 at 5:29 AM, Jassi Brar <jaswinder.singh@...aro.org> wrote:
>> On 18 October 2012 12:15, Huang Shijie <b32955@...escale.com> 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 ...
>>>
>>> But with this patch, we only need to enable/disable dma clock one time, just
>>> at we select the nand chip.
>>>
>> If the clock is the dmac's property (not channels'), the toggling
>> seems too aggressive.
>> You could try using runtime_suspend/resume for clock
>> disabling/enabling. How about employing autosuspend with a few ms
>> delay?
>
> Yes, employing the autosuspend is workable method too.o
> But it's a little more complicated then this patch. You have to create
> a thread or workqueue to
> do the job.
>
I am not sure why you would need to create any thread/workqueue ?
Just pm_runtime_put_autosuspend() in drivers/ to see how it works.
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