linux-kernel - Re: [PATCH] dma: tegra: enable/disable dma clock

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Date:	Fri, 20 Jul 2012 12:10:45 +0530
From:	Vinod Koul <vinod.koul@...ux.intel.com>
To:	Laxman Dewangan <ldewangan@...dia.com>
Cc:	"dan.j.williams@...el.com" <dan.j.williams@...el.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Stephen Warren <swarren@...dia.com>,
	"linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>
Subject: Re: [PATCH] dma: tegra: enable/disable dma clock

On Fri, 2012-07-20 at 11:34 +0530, Laxman Dewangan wrote:
> On Friday 20 July 2012 11:32 AM, Vinod Koul wrote:
> > On Wed, 2012-07-18 at 14:26 +0530, Laxman Dewangan wrote:
> >> Enable the DMA clock when registering DMA driver and
> >> disable clock when removing the DMA driver.
> >>
> >> The failure was observed on Tegra20 based system by
> >> Stephen Warren. However, it is working fine on tegra30
> >> based system and probably becasue uboot enable the clock
> >> on Tegra30.
> > Don't know much about the clock subsystem, but shouldn't the clocks be
> > enabled runtime..... when you actually start using the controller??
> 
> Yes, this is the plan and I am going to implement dynamic clock 
> management but it will take some time to complete and have testing 
> throughly. some of the issue which I am seeing and analyzing on this 
> approach is that
>   - clock disable can be called from isr context or not?
>   - how much complexity will be added in dma driver to implement this?
>   - What power benefit we will get on this.
>   - Is there any user which generally use dma and always keep dma on 
> requested state like UART RX always ready etc.
> 
> So keeping that in mind, it will take some time to complete this and 
> meanwhile, I want to make it work for Tegra20 based system which is not 
> working on current driver.
> and hence this is the change.
for a start you can move these to .alloc_chan_resources
and .free_chan_resources while doing a  refcount.
Further optimization can be done by moving this to tx_submit and
tasklet.

-- 
~Vinod

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