linux-kernel - Re: [PATCH v8 3/5] amba: Don't unprepare the clocks if device driver wants IRQ safe runtime PM

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Date:	Tue, 4 Nov 2014 17:19:13 +0100
From:	Ulf Hansson <ulf.hansson@...aro.org>
To:	"Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:	Alan Stern <stern@...land.harvard.edu>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	Krzysztof Kozlowski <k.kozlowski@...sung.com>,
	Len Brown <len.brown@...el.com>, Pavel Machek <pavel@....cz>,
	Jonathan Corbet <corbet@....net>,
	Dan Williams <dan.j.williams@...el.com>,
	Vinod Koul <vinod.koul@...el.com>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	"linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	dmaengine@...r.kernel.org, Lars-Peter Clausen <lars@...afoo.de>,
	Michal Simek <michal.simek@...inx.com>,
	Kyungmin Park <kyungmin.park@...sung.com>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>
Subject: Re: [PATCH v8 3/5] amba: Don't unprepare the clocks if device driver
 wants IRQ safe runtime PM

On 4 November 2014 14:59, Rafael J. Wysocki <rjw@...ysocki.net> wrote:
> On Tuesday, November 04, 2014 10:11:35 AM Ulf Hansson wrote:
>> On 4 November 2014 02:57, Rafael J. Wysocki <rjw@...ysocki.net> wrote:
>> > On Monday, November 03, 2014 10:41:02 AM Alan Stern wrote:
>> >> On Mon, 3 Nov 2014, Russell King - ARM Linux wrote:
>> >>
>> >> > That makes it pretty horrid from the point of view of having bus
>> >> > management code, because we now have the management of the bus clock
>> >> > split between the bus layer and the device driver.
>> >> >
>> >> > This is /really/ a problem for runtime PM.  Runtime PM permits there
>> >> > to be a bus layer involved - and runtime PM can also be coupled up
>> >> > to PM domains as well.  For all this stuff, the context which the
>> >> > callbacks are called in depends on whether the driver itself has
>> >> > marked the device as having IRQ-safe callbacks.
>> >> >
>> >> > That's fine, but the bus and PM domain level code then /really/ needs
>> >> > to know what context they're being called in, so they know whether
>> >> > they can sleep or not, or they must to be written to always use
>> >> > non-sleeping functions so they work in both contexts.  If we assume
>> >> > the former, then that implies that the irq-safe flag must never change
>> >> > state between a suspend and a resume.
>> >>
>> >> If a bus subsystem or PM domain is going to allow its drivers to choose
>> >> between IRQ-safe and non-IRQ-safe runtime PM, then it is up to the
>> >> subsystem to come up with a way for drivers to indicate their choice.
>> >>
>> >> I tend to agree with Rafael that testing dev->power.irq_safe should be
>> >> good enough, with no real need for a wrapper.  But the subsystem can
>> >> use a different mechanism if it wants.
>> >>
>> >> Bear in mind, however, that once the irq_safe flag has been set, the
>> >> runtime PM core offers no way to turn it off again.
>> >
>> > There is a problem with it, though.  Say, a driver handles a device that
>> > may or may not be in a power domain.  Or in other words, the power domain
>> > the device is in may or may not be always on.  If the domain is always on,
>> > the runtime PM callbacks are IRQ-safe (they depend on the driver only).
>> > If it isn't, they may not be IRQ-safe.  How's the driver going to decide
>> > whether or not to set power.irq_safe?
>>
>> From my point of view; the decision whether the driver will set the
>> IRQ safe flag is in principle a software design choice.
>>
>> Currently genpd isn't able to power off, if one of its devices are IRQ
>> safe configured. That's a limitation in genpd which we need to fix and
>> it's on my TODO list.
>>
>> My point is thus, I don't think the driver should care about PM
>> domains at all regarding using the IRQ safe option. Does that make
>> sense?
>
> Yes, it does, and that's the heart of the problem above.  The driver should
> not care about wherther or not the device is in a power domain, but it needs
> to know that when deciding whether or not to set power.irq_safe.  Catch 22.

Why is it catch22? The problem is supposed to be fixed in the generic
PM domain. How we do that is a different question.

Until genpd get fixed, the driver can still keep using irq_safe if
they want to. It will only lead to limitations if the device is
attached to a genpd.

Kind regards
Uffe
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