Date:	Thu, 12 Nov 2015 13:35:25 +0000
From:	Jon Hunter <jonathanh@...dia.com>
To:	Lars-Peter Clausen <lars@...afoo.de>,
	Grygorii Strashko <grygorii.strashko@...com>,
	Thomas Gleixner <tglx@...utronix.de>
CC:	Jason Cooper <jason@...edaemon.net>,
	Marc Zyngier <marc.zyngier@....com>,
	Stephen Warren <swarren@...dotorg.org>,
	Thierry Reding <thierry.reding@...il.com>,
	Kevin Hilman <khilman@...nel.org>,
	Geert Uytterhoeven <geert@...ux-m68k.org>,
	LKML <linux-kernel@...r.kernel.org>,
	<linux-tegra@...r.kernel.org>,
	Soren Brinkmann <soren.brinkmann@...inx.com>,
	Linus Walleij <linus.walleij@...aro.org>,
	Alexandre Courbot <gnurou@...il.com>
Subject: Re: [RFC PATCH 1/2] genirq: Add runtime resume/suspend support for
 IRQ chips


On 12/11/15 13:20, Lars-Peter Clausen wrote:
> On 11/12/2015 11:59 AM, Jon Hunter wrote:
>>
>> On 11/11/15 15:41, Grygorii Strashko wrote:
>>> On 11/11/2015 12:13 PM, Jon Hunter wrote:
>>>>
>>>> On 10/11/15 18:07, Lars-Peter Clausen wrote:
>>>>> On 11/10/2015 05:47 PM, Grygorii Strashko wrote:
>>>>> [...]
>>>>>>> I was trying to simplify matters by placing the resume call in
>>>>>>> __setup_irq() as opposed to requested_threaded_irq(). However, the would
>>>>>>> mean the resume is inside the bus_lock and may be I should not assume
>>>>>>> that I can sleep here.
>>>>>>>
>>>>>>>> Can you folks please agree on something which is correct and complete?
>>>>>>>
>>>>>>> Soren I am happy to defer to your patch and drop this. My only comment
>>>>>>> would be what about the request_percpu_irq() path in your patch?
>>>>>>>
>>>>>>
>>>>>> I have the same comment here as I asked Soren:
>>>>>> 1) There are no restrictions to call irq set_irq_type() whenever,
>>>>>> as result HW can be accessed before request_x_irq()/__setup_irq().
>>>>>> And this is used quite widely now :(
>>>>>>
>>>>>
>>>>> Changing the configuration of a resource that is not owned seems to be
>>>>> fairly broken. In the worst case this will overwrite the configuration that
>>>>> was set by owner of the resource.
>>>>>
>>>>> Especially those that call irq_set_irq_type() directly before request_irq(),
>>>>> given that you supply the trigger type to request_irq() which will make sure
>>>>> that there are no conflicts and the configure.
>>>>>
>>>>> This is a bit like calling gpio_set_direction() before you call
>>>>> gpio_request(), which will also have PM issues.
>>>>
>>>> Yes, I agree that this does sound a bit odd, but ...
>>>>
>>>>>> For example, during OF boot:
>>>>>>
>>>>>> [a]  irq_create_of_mapping()
>>>>>>     - irq_create_fwspec_mapping()
>>>>>>       - irq_set_irq_type()
>>>>
>>>> The above means that if someone calls of_irq_get() (or
>>>> platform_get_irq()), before request_irq(), then this will call
>>>> irq_create_of_mapping() and hence, call irq_set_irq_type. So should
>>>> irq_create_fwspec_mapping() be setting the type in the first place? I
>>>> can see it is convenient to do it here.
>>>
>>> In general there is another option - save OF-flags and pass them to
>>> __setup_irq() where they can be processed.
>>
>> Right, we could look at doing something like this.
>>
>>>>>> or
>>> [b]
>>>>>> 	irq_set_irq_type(irq, IRQ_TYPE_LEVEL_HIGH);
>>>>>> 	irq_set_chained_handler(irq, mx31ads_expio_irq_handler);
>>>
>>> option: add "flag" parameter to irq_set_chained_handler
>>>
>>>>>>
>>>>>> or
>>> [c]
>>>>>> 	irq_set_irq_type(alarm_irq, IRQ_TYPE_EDGE_BOTH);
>>>>>> 	err = devm_request_irq(&pdev->dev, alarm_irq, fan_alarm_irq_handler,
>>>>>> (there are ~200 occurrences of irq set_irq_type in Kernel)
>>>>>>
>>>>>> 2) if i'm not wrong, the same is valid for irq_set_irq_wake() and irq_set_affinity()
>>>>>>
>>>>>> I'm not saying all these code is correct, but that what's now in kernel :(
>>>>>> I've tried to test Soren's patch with omap-gpio and immediately hit case [a] :.(
>>>>>
>>>>> All functions for which are part of the public API and for which it is legal
>>>>> to call them without calling request_irq() (or similar) first will need to
>>>>> have pm_get()/pm_put().
>>>>
>>>> Right. May be we can look at the various entry points to the chip
>>>> operators to get a feel for which public APIs need to be handled.
>>>
>>>
>>> Seems yes. But we need to be very careful with this, some of functions could be
>>> called recursively (nested), like:
>>> [d]
>>> static int pcf857x_irq_set_wake(struct irq_data *data, unsigned int on)
>>> {
>>> 	...
>>> 	error = irq_set_irq_wake(gpio->irq_parent, on);
>>>
>>>
>>> Personally, I have nothing against irq_pm_(get|put) :) and thought about similar things
>>> when tried to solve the same problem for omap-gpio driver.
>>> But :(, I have to fall back to irq_bus_lock/sync_unlock, because of [a,b,c] - all above
>>> APIs surrounded by chip_bus_lock/chip_bus_sync_unlock. ([d] - I've not hit it just because
>>> I was lucky).
>>
>> I had a quick peek at the omap-gpio driver and I see that internally you
>> are using the gpio ref-count to manage RPM and use the bus-lock hooks to
>> invoke RPM.
>>
>> This can definitely be complex when considering all the potential paths,
>> but I think that we need to a look at this from a chip-ops perspective
>> because only the chip knows if it is accessible or not. That said, what
>> we need to assess is:
>>
>> 1. Which chip-ops should ONLY be called after an IRQ has been allocated
>>    (eg, enable/disable, mask/unmask, type, etc). These chip-ops should
>>    not try to control the chip PM, but should possibly WARN if called
>>    when  the chip is not accessible.
>> 2. For chip-ops that may be called without allocating an IRQ (eg.
>>    bus_lock, irq_suspend, etc), can these be called from an atomic
>>    context? If they might be called from an atomic context then these
>>    are the chip-ops which will cause problems as we cannot guarantee
>>    that all IRQ chips can support irq-safe RPM.
> 
> They can't. chip_bus_lock() can sleep, so anything that locks the bus can't
> be called from atomic context.

Sorry, what can't? Yes I understand that we cannot call anything that
locks the bus from an atomic context.

> One easy way out might be to always call pm_get/pm_but from
> bus_lock,/bus_unlock. This way the chip is guaranteed to be powered up when
> accessed happens. In addition pm_get is called when the IRQ is request and
> pm_put is called when the IRQ is release, this is to ensure the chip stays
> powered when it is actively monitoring the IRQ lines.

Yes I had thought about that, but it is not quite that easy, because in
the case of request_irq() you don't want to pm_put() after the
bus_unlock(). However, the bus_lock/unlock() are good indicators of
different paths.

Cheers
Jon



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