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

[Ulf Hansson]

On 4 November 2014 02:57, Rafael J. Wysocki <rjw@xxxxxxxxxxxxx> 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?

Kind regards
Uffe
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