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Date: Mon, 3 Nov 2014 10:41:02 -0500 (EST)
From: Alan Stern <stern@rowland.harvard.edu>
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To: Russell King - ARM Linux <linux@arm.linux.org.uk>
cc: Krzysztof Kozlowski <k.kozlowski@samsung.com>,
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        Ulf Hansson <ulf.hansson@linaro.org>, <linux-pm@vger.kernel.org>,
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        <dmaengine@vger.kernel.org>, Lars-Peter Clausen <lars@metafoo.de>,
        Michal Simek <michal.simek@xilinx.com>,
        Kyungmin Park <kyungmin.park@samsung.com>,
        Marek Szyprowski <m.szyprowski@samsung.com>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Subject: Re: [PATCH v8 3/5] amba: Don't unprepare the clocks if device driver
 wants IRQ safe runtime PM
In-Reply-To: <20141103100442.GB4042@n2100.arm.linux.org.uk>
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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