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

[Andy Shevchenko <andriy.shevchenko@linux.intel.com>]

On Sun, 2014-01-26 at 16:47 +0530, Vinod Koul wrote:

> > > > > 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 :)
> > 
> > ...as I already wrote before we have no parent-child relationship
> > between DMA and, for example, SPI. That means we may possible have the
> > case when SPI's .suspend() will be called later than DMA's one.
> > 
> > > > It seems for me all DMA drivers that are using
> > > > system .suspend()/.resume() are potentially buggy.
> > > Yup!
> > 
> > So, we have to decide what to do with them. .suspend_late() still seems
> > for me not the best approach. *Or* we have to check for ongoing
> > transaction and do something with it. *Or* just shut down the device and
> > rely on DMA transaction initiator that it handles the terminated
> > transaction properly.
> 
> As you clearly said, we dont have a parent-child relatation though we have big
> dependency. I think this is true for DMA clients, i ran into similar situation
> with i2c few days back!
> 
> So only think which can give us a good system behaviour would be clients getting
> suspended first and then then service providing subsystems.

Agree.

>  (same reason why we
> do dma driver loading and init much before others drivers)

Yes, it would be done via deferred probe.

> So yes in the .suspend callback of the client, it needs to
> 1) abort any transactions it has
> 2) make the client quiscent
> 
> then the .late_suspend kicks in and suspend the core drivers like dma.
> 
> This is how it has worked reliably for me in production systems. I am all ears
> if we have a better and cleaner apprach to this problem :)

Yes, summarize everything we discussed we have to:
 - provide suspend_late, resume_early callbacks in the DMA driver
instead of *_noirq versions
 - ensure that all clients on our platforms follow the described
scenario

Chiau Ee, I think you may to change your patch accordingly.


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