linux-kernel - Re: [PATCH 1/6] dmaengine: tegra-apb: Correct runtime-pm usage

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Message-ID: <1564097.buIA8Wdn8f@vostro.rjw.lan>
Date:	Thu, 05 Nov 2015 03:15:18 +0100
From:	"Rafael J. Wysocki" <rjw@...ysocki.net>
To:	Kevin Hilman <khilman@...nel.org>,
	Vinod Koul <vinod.koul@...el.com>
Cc:	Jon Hunter <jonathanh@...dia.com>,
	Laxman Dewangan <ldewangan@...dia.com>,
	Stephen Warren <swarren@...dotorg.org>,
	Thierry Reding <thierry.reding@...il.com>,
	Alexandre Courbot <gnurou@...il.com>,
	dmaengine@...r.kernel.org, linux-tegra@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/6] dmaengine: tegra-apb: Correct runtime-pm usage

On Wednesday, November 04, 2015 08:59:43 AM Kevin Hilman wrote:
> Vinod Koul <vinod.koul@...el.com> writes:
> 
> > On Tue, Nov 03, 2015 at 01:25:09PM -0800, Kevin Hilman wrote:
> >> >>>>>  	/* Enable clock before accessing register */
> >> >>>>> -	ret = tegra_dma_runtime_resume(dev);
> >> >>>>> +	ret = pm_runtime_get_sync(dev);
> >> >>>>
> >> >>>> If you are runtime suspended then core will runtime resume you before
> >> >>>> invoking suspend, so why do we need this
> >> >>>
> >> >>> Is this change now in the mainline? Do you have commit ID for that?
> >> >>>
> >> >>> I recall the last time we discussed this that Rafael said that they were
> >> >>> going to do that, but he said as a rule of thumb if you need to resume
> >> >>> it, resume it [0].
> >> >> 
> >> >> IIRC this has been always the behaviour, at least I see this when I test the
> >> >> devices
> >> >
> >> > I have been doing some testing today and if the DMA is runtime
> >> > suspended, then I don't see it runtime resumed before suspend is called.
> >> >
> >> > Can you elborate on "at least I see this when I test the devices"? What
> >> > are you looking at? Are you using kernel function tracers in some way?
> >> 
> >> The PM core does a _get_noresume()[1] which tries to prevent runtime
> >> suspends *during* a system suspend.  However, the PM core should not be
> >> doing an actual runtime resume of the device, so if the device is
> >> already runtime suspended, it will not be runtime resumed by the core,
> >> so if the driver needs it to be runtime resumed, it needs to do it
> >> itself.
> >
> > + Rafael
> >
> > This is contrariry to what I see, If my driver is runtime suspended and on
> > suspend, it gets runtime resumed and then suspended
> 
> Since I was late to the thread, can you explain what kind of driver and
> on what bus type you're seeing this behavior?
> 
> It could be that your bus-type is doing something, but I don't think it
> should be the PM core.

Right.

Bus types do that, the core doesn't.  The ACPI PM domain does that too
for some devices.

So Vinod, more details, please.

Thanks,
Rafael

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