Re: [PATCH] dma: dw: Add suspend and resume handling for PCI mode DW_DMAC.

[Vinod Koul <vinod.koul@intel.com>]

On Mon, Jan 20, 2014 at 06:17:56PM +0530, Shevchenko, Andriy wrote:
> 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.
well how is that. It will gaurantee that there wont be interrupt. User can still
submit a transaction or another transaction will be in progress...

> > 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.
Well client is already suspended via .suspend. So where is the transaction :)
> 
> It seems for me all DMA drivers that are using
> system .suspend()/.resume() are potentially buggy.
Yup!

-- 
~Vinod