Re: [PATCH] dma: add new DMA control commands

From: Huang Shijie
Date: Thu Oct 18 2012 - 11:18:46 EST

On Thu, Oct 18, 2012 at 5:29 AM, Jassi Brar <jaswinder.singh@xxxxxxxxxx> wrote:
> On 18 October 2012 12:15, Huang Shijie <b32955@xxxxxxxxxxxxx> wrote:
>> ä 2012å10æ18æ 14:18, Vinod Koul åé:
>>> 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.

What's more, I think other DMA engine may also meets the same issue as
the mxs-dma, such as the imx-sdma.
It's somehow a common issue to disable the clocks when no one use the
DMA engine.
Do you also suggest employing the autosuspend? Why not use a simple
way to handle this issue?
When the dma device want to use a DMA engine, it just needs to issue a
DMA_START command.

Huang Shijie
Huang Shijie
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