Re: [PATCH v2 5/7] dmaengine: dw: platform: power on device on shutdown

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

On Thu, 2015-11-26 at 23:11 +0530, Vinod Koul wrote:
> On Thu, Nov 26, 2015 at 07:24:48PM +0200, Andy Shevchenko wrote:
> > On Thu, 2015-11-26 at 22:31 +0530, Vinod Koul wrote:
> > > On Thu, Nov 26, 2015 at 05:19:11PM +0200, Andy Shevchenko wrote:
> > > > We have to call dw_dma_disable() to stop any ongoing transfer.
> > > 
> > > Ok
> > > 
> > > > On some platforms we can't do that since DMA device is powered
> > > > off.
> > > 
> > > Umm, you said we have ongoing transfer which means DMA should be
> > > on..!!
> > 
> > Yes, that's true for HSW/BDW and non-affected BYT/CHT.
> 
> Can you please explain even when DMA is in use how can device be
> powered
> off? That does not sound right to me. 

It can't, but the problem is we can't distinguish that in this routine!
We simple do *not* know the actual power state of DMA.

These calls *ensures* that DMA is powered on. Yes, the call to
dw_dma_off() when it used to be powered off sounds silly, I agree,
though I see no upstreamable (non-hackish) solution for that.

Previously I proposed to remove .shutdown hook completely, you were
objecting.

> Is this on GP DMA on BYT/CHT or
> something else?

Correct. Affected platforms are BYT-T and some or all of BSW/CHT
depending on firmware in use.

> 
> > Like I mentioned here is no possibility to know which platform we
> > run
> > on.
> > 
> > Would you like to test this on a real device? We can provide you a
> > login.
> > 
> > > 
> > > > Moreover we have no
> > > > possibility at that point to check if the platform is affected
> > > > or
> > > > not. That's
> > > > why we call pm_runtime_get_sync() / pm_runtime_put()
> > > > unconditionally. On the
> > > > other hand we can't use pm_runtime_suspended() because runtime
> > > > PM
> > > > framework is
> > > > not fully used by the driver.
> > > 
> > > Shouldn't that be fixed?
> > 
> > Do you have any solution how?
> > 
> > Rough approach is to turn on it on channel allocation and turn off
> > on
> > freeing resources. The obvious downside of this solution is power
> > consumption of idling device.
> 
> But in that case, the clients should not hold ref of dma chan when
> idle and
> allocate only when required which is a resonable expectation

There is not the case for few drivers. At least for us it's spi-pxa2xx
one. It requires channels on its ->probe() stage. Jarkko is Cc'ed here,
you may ask him as he is our maintainer for the SPI.

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