RE: [PATCH] dma: dw: Add suspend and resume handling for PCI modeDW_DMAC.

[Chew, Chiau Ee]

> -----Original Message-----
> From: Andy Shevchenko [mailto:andriy.shevchenko@xxxxxxxxxxxxxxx]
> Sent: Monday, January 27, 2014 6:17 PM
> To: Koul, Vinod
> Cc: Andy Shevchenko; Chew, Chiau Ee; Viresh Kumar; Williams, Dan J;
> dmaengine@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx
> Subject: Re: [PATCH] dma: dw: Add suspend and resume handling for PCI mode
> DW_DMAC.
> 
> 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.

Ok. Noted

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

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