[RFC] gpio: about the need to manage irq mapping dynamically.

[Grygorii Strashko]

On 06/27/2017 01:25 PM, Jerome Brunet wrote:
> On Tue, 2017-06-27 at 12:49 -0500, Grygorii Strashko wrote:
>>
>> On 06/22/2017 09:25 AM, Jerome Brunet wrote:
>>> On Wed, 2017-06-21 at 22:50 +0200, Linus Walleij wrote:
>>>> On Tue, Jun 20, 2017 at 7:23 PM, Jerome Brunet <jbrunet at baylibre.com>
>>>> wrote:
>>>>> On Tue, 2017-06-20 at 18:37 +0200, Linus Walleij wrote:
>>>>>> Eventually gpio_to_irq() should be DELETED and replaced in full with
>>>>>> the prepare/unprepare calls.
>>>>>
>>>>> Woahh, that's not what I meant. gpio_to_irq should stay. Getting rid of
>>>>> it
>>>>> would
>>>>> be a mess and it is a useful call.
>>>>>
>>>>> The gpio_irq_prepare is meant so that the consumer can tell the gpio
>>>>> driver
>>>>> it
>>>>> will want to get irq from a particular gpio at some point.
>>>>>
>>>>> IOW, it's the consumer saying to the gpio driver "please do whatever you
>>>>> need to
>>>>> do, if anything, so this gpio can generate an interrupt"
>>>>>
>>>>> This is a much simpler change. Using devm, all we need is to put a
>>>>> devm_gpio_irq_prepare(<gpio_num>) in the probe of the drivers using
>>>>> gpio_to_irq.
>>>>>
>>>>> Mandating call to gpio_irq_prepare before any call to gpio_to_irq will
>>>>> be
>>>>> fairly
>>>>> easy.
>>>>
>>>> So why can't we just return the IRQ from prepare() and be done with it
>>>
>>> We can return it here as well, it's actually a good idea. New drivers could
>>> just
>>> use that one if they are keeping track of the irq number.
>>>
>>>> instead of having two calls? (Plus a third eventual unprepare()).
>>>>
>>>> Clocks, regulators and godknowswhat is managed by two symmetrical
>>>> calls, so why shouldn't GPIO IRQs be?
>>>
>>> The approach is exactly the same as what we trying to follow in the irq
>>> framework:
>>>
>>> framework     | irq                 | gpio
>>> -----------------------------------------------------
>>> index         | hwirq               | gpio_num
>>> creation      | irq_create_mapping  | gpio_irq_prepare   ?
>>> removal       | irq_dispose_mapping | gpio_irq_unprepare ?
>>> (fast) lookup | irq_find_mapping    | gpio_to_irq
>>>
>>> We are going to have at lookup function somehow, why not expose it ?
>>>
>>> Another reason for keeping gpio_to_irq is that many existing drivers using
>>> the
>>> callback don't keep track of their irq number, just the gpio. For every irq
>>> operation, they call gpio_to_irq. Things like this:
>>>
>>> * irq_enable(gpio_to_irq(<gpio_num>))
>>> * irq_release(gpio_to_irq(<gpio_num>))
>>> * etc ...
>>>
>>> It's a bit lazy maybe, but I don't think there is anything utterly wrong
>>> with
>>> it.
>>>
>>> Getting rid of gpio_to_irq would mean reworking all those drivers so they
>>> keep
>>> track of their irq number. I think this would be a mess for a very little
>>> gain.
>>>
>>> Also, except for the 26 gpio drivers I have listed, the rest should already
>>> be
>>> implementing what qualify as a "lookup" function in gpio_to_irq. I don't
>>> think
>>> we should by modifying every single gpio driver when there is a solution to
>>> 'surgically' address the matter.
>>>
>>> The series would already affect a significant amount of drivers, I'm trying
>>> to
>>> keep it as simple an contained as possible.
>>>
>>> If that is OK with you, I could send an RFC implementing the idea but fixing
>>> only 1 gpio driver and 2 or 3 consumer. We would have actual code to discuss
>>> on,
>>> it might be easier.
>>>
>>
>> I'd like to add my 5 cents here :)
>> 1) "To create the mapping in the gpio_to_irq. Linus, you pointed out that this
>> is not
>>   allowed as gpio_to_irq is callable in irq context, therefore it should not
>> sleep.
>>   Actually 3 drivers [2] are calling gpio_to_irq in irq handlers."
>>
>> It's not allowed to call gpio_to_irq() from IRQ handlers, as mappings should
>> already exist at
>> that time and It might require to do sleepable calls to crate IRQ mappings and
>> configure HW.
>>
>> Drivers, pointed out in first e-mail, should use other APIs in their IRQ
>> handlers:
>> drivers/gpio/gpio-ep93xx.c
>> ^ direct call to ep93xx_gpio_to_irq()
>> * drivers/gpio/gpio-pxa.c
>>   ^ use irq_find_mapping()
>> * drivers/gpio/gpio-tegra.c
>>   ^ use irq_find_mapping()
>>
>> Also note, IRQ mappings can be created as dynamically (each
>> time  gpio_to_irq() is called)
> 
> Not according to a previous reply from Linus. Right or wrong, it would have made
> my life a lot easier if it was OK :)
> 
>>   as
>> statically (in probe). The last approach is widely used in gpio drivers due to
>> compatibility and
>> legacy reasons.
> 
> Agreed this is the current situation.
> 
>>
>> 2) As per above I do not really understand why gpio_irq_prepare() is required.
> 
> I'd like to point you the thread which initially triggered this rfc:
> https://patchwork.ozlabs.org/patch/684208/
> 
> At the time Linus strongly rejected the idea of calling  irq_create_mapping (or
> any sleeping functions) in gpio_to_irq: please see the reply from Oct 26, 2016
> (sorry for quoting such an old discussion but this is really the starting point)
> 
> * Me: There is really a *lot* of gpio drivers which use irq_create_mapping in
> the to_irq callback, are these all wrong ?
> * Linus: Yes they are all wrong. They should all be using irq_find_mapping().
> 
> * Me: If this should not be used, what should we all do instead ?
> * Linus: Call irq_create_mapping() in some other place.
> 
> gpio_prepare_irq is a proposition for this 'other place'.

And my opinion is still the same here - It should be perfectly valid to create
mappings from gpio_to_irq() to handle properly orthogonality of gpiochip and
gpio-irqchip functionality and satisfy SPARSE_IRQ goal (allocate Linux virq and
irq descriptors on demand).

The valid use case for gpio_to_irq() is below (and only one valid use-case):
- consumer driver probe() (or other init function):
 gpio = [devm_]gpiod_get()
 irq =  gpio_to_irq(gpio)
	|- in general case -> irq_create_mapping()
 request_irq(irq,...)

>From another side, driver may request GPIO IRQ without even knowing about gpiochip
- consumer driver probe() (or other init function):
 irq = platform_get_irq()/of_irq_get()
 	|- in general case -> irq_create_mapping()
 request_irq(irq,...)

In both cases, IRQ mapping is expected to be created by corresponding IRQ domain.
More over, drivers can share GPIO IRQ and in this case first call to
irq_create_mapping() will actually create mapping while second will be transformed to
irq_find_mapping().

> 
>>
>> 3) As per problem description irq_dispose_mapping() equivalent for GPIO IRQs
>> might be required,
>> but what is the real use-case? Modules reloading or unloading one module and
>> loading another instead?
>> Usually GPIO and IRQ configuration is static and defined in DT, so IRQ mapping
>> created by first call to
>> gpio_to_irq()/platform_get_irq()/of_irq_get() and any subsequent calls just
>> re-use already created mapping.
> 
> Providing that you always create mapping for the same pins, yes
> What happens when gpio irq (the pin) is different each time ? and you exhaust
> the ressource ?

So, do you expect use-case like:
- request GPIO IRQs for pins A1..A8
- free GPIO IRQs A5..A8
- request GPIO IRQs for pins B1..A4
...
(you have 8 gpio irqs as per your description)

This is not smth really common and that why I'm asking ;) and that's why nobody really care
about GPIO IRQs un-mapping. It's kinda expected to plan GPIO IRQ usage very carefully hence
you have only 8 of them.

> 
> It is a corner case, but possible isn't it ? With the gpio char driver
> interface  maybe. You would have to reboot to flush the old mappings and
> continue, right ?

true, but no complains till now ;) And, as I said, It might be reasonable to
have gpio_irq_unprepare() - if you have real use-case description ;)
But note, it's unclear how to integrate it? Will it be required to modify all drivers
which are using  gpio_to_irq() (not good) or it can be integrated in gpiolib and triggered automatically (good)?

> 
> You could also fail to set the trigger type (which you won't be able to set in
> gpio_to_irq). If so, shouldn't you release the mapping ? (that's real use-case
> we are having by the way).

Sry, I didn't get this. IRQ type is set in request_irq() or by irqd_set_trigger_type().
Its actually expected to be reset (configured to safe state) in gpio-irqchip->irq_shutdown()
callback.

Also, It might be helpful to take a look on drivers/irqchip/irq-crossbar.c.



-- 
regards,
-grygorii