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:	Mon, 3 Nov 2014 10:41:02 -0500 (EST)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
cc:	Krzysztof Kozlowski <k.kozlowski@...sung.com>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	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>,
	Ulf Hansson <ulf.hansson@...aro.org>,
	<linux-pm@...r.kernel.org>, <linux-doc@...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 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.

Alan Stern

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