linux-kernel - Re: [PATCH] dma: dw: Add suspend and resume handling for PCI mode DW

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Date:	Mon, 20 Jan 2014 12:47:56 +0000
From:	"Shevchenko, Andriy" <andriy.shevchenko@...el.com>
To:	"Koul, Vinod" <vinod.koul@...el.com>
CC:	Andy Shevchenko <andriy.shevchenko@...ux.jf.intel.com>,
	"Chew, Chiau Ee" <chiau.ee.chew@...el.com>,
	Viresh Kumar <viresh.linux@...il.com>,
	"Williams, Dan J" <dan.j.williams@...el.com>,
	"dmaengine@...r.kernel.org" <dmaengine@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] dma: dw: Add suspend and resume handling for PCI mode
 DW_DMAC.

On Mon, 2014-01-20 at 14:55 +0530, Vinod Koul wrote:
> On Thu, Dec 19, 2013 at 12:51:29PM +0200, Andy Shevchenko wrote:
> > On Wed, 2013-12-18 at 21:19 +0530, Vinod Koul wrote:
> > > On Mon, Dec 16, 2013 at 01:51:47PM +0530, Chew, Chiau Ee wrote:
> > 
> > > > As mentioned by Andy, we are using *_noirq  verion of suspend/resume PM
> > > > callback whereby the callbacks would be executed after IRQ handlers have been
> > > > disabled. If using SET_SYSTEM_SLEEP_PM_OPS, it would be the normal
> > > > suspend/resume PM callback. Looking at the Desginware DMAC platform code
> > > > (drivers/dma/dw/platform.c), it is using the *_noirq suspend/resume PM
> > > > callback. Is it advisable to use the normal suspend/resume PM callback instead
> > > > of *_noirq suspend/PM callback? 
> > > 
> > > i dont see a reason why we need the noirq versions
> > 
> > Okay. I imagine the following use case.
> > 
> > For example we have compiled in DMA driver (dw_dmac) along with, for
> > example, SPI driver.
> > 
> > System was scheduled to go sleep.
> > 
> > An order of calling IIUC might be DMA first, then SPI (since they are
> > not in parent / child relations).
> > 
> > What was happened when SPI would like to do a DMA transfer and DMA is
> > going to sleep? I'm trying to understand if this is a case.
> In that case how does no irq version help us?

It guarantees that we have no user of DMA anymore, since there is no
interrupt going on.

> For these cases, I have been using suspend_late. Since the dmaengine driver is
> providing service to other clients (SPI), it needs to esnure that it suspends
> after SPI using suspend_late and resume using resume_early. That way dma is
> availble whenever the client is active

suspend_late is working in context that interrupt handler may be
invoked. Thus, to have DMA driver be properly shut down we have to
wait / terminate possible ongoing transfer.

It seems for me all DMA drivers that are using
system .suspend()/.resume() are potentially buggy.

> 
> --
> ~Vinod
> 
> > 
> > > > > How about SET_SYSTEM_SLEEP_PM_OPS instead?
> > > > 
> > > > So, we are using *_noirq versions of the functions here. What happened when we switch to normal ones? Any side effects?
> > 
> > 
> > 
> > -- 
> > Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
> > Intel Finland Oy
> > 
> 

-- 
Andy Shevchenko <andriy.shevchenko@...el.com>
Intel Finland Oy
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