Date:   Mon, 6 Feb 2017 17:01:58 +0100
From:   Fabrice Gasnier <fabrice.gasnier@...com>
To:     Jonathan Cameron <jic23@...nel.org>,
        Linus Walleij <linus.walleij@...aro.org>
CC:     Russell King <linux@...linux.org.uk>,
        Rob Herring <robh+dt@...nel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-iio@...r.kernel.org" <linux-iio@...r.kernel.org>,
        Mark Rutland <mark.rutland@....com>,
        Maxime Coquelin <mcoquelin.stm32@...il.com>,
        Alexandre TORGUE <alexandre.torgue@...com>,
        Lars-Peter Clausen <lars@...afoo.de>,
        Hartmut Knaack <knaack.h@....de>,
        Peter Meerwald <pmeerw@...erw.net>,
        Benjamin Gaignard <benjamin.gaignard@...aro.org>,
        Benjamin Gaignard <benjamin.gaignard@...com>,
        <tglx@...utronix.de>
Subject: Re: [PATCH v2 4/5] iio: trigger: add support for STM32 EXTI triggers

On 02/04/2017 12:39 PM, Jonathan Cameron wrote:
> On 03/02/17 19:40, Linus Walleij wrote:
>> On Mon, Jan 30, 2017 at 3:33 PM, Fabrice Gasnier <fabrice.gasnier@...com> wrote:
>>
>>> EXTi[0..15] gpio signal can be routed internally as trigger source for
>>> ADC or DAC conversions. Configure them as interrupts to configure
>>> trigger path in HW.
>>>
>>> Note: interrupt handler isn't required here, and corresponding interrupt
>>> can be kept masked at exti controller level.
>>>
>>> Signed-off-by: Fabrice Gasnier <fabrice.gasnier@...com>
>>
>> But I see nothing STM32-specific about this driver?
>>
>> I think we should do everone a service and just create
>> drivers/iio/trigger/gpio-trigger.c
>>
>> I wondered before why we don't have a generic GPIO IIO trigger,
>> it seems like such an intuitive and useful thing to have.
> We do, it just got renamed at some point a while back to be
> iio-trig-interrupt after it became clear that it didn't need
> to be a gpio either - just an interrupt.  Can't remember which
> part provided a non gpio interrupt pin and hence drove that
> change.  Was quite a while back!
> d4fd73bf25c3aafc891ef4443fc744d427ec1df0 specifically in 2013
>
> Handling of the gpio stuff should be handled in the interrupt
> description itself.
>
> However, it's a bit different - in that in the below it
> would be the equivalent of triggering on the unused exti
> interrupt rather than on the end of conversion.
>
> In this case, because of the hardware linkage we can effectively
> skip the first interrupt.
>
> Arguably to make this a general purpose trigger we should enable
> that interrupt if anything other than the STM devices that can
> use it in hardware are hooked on to it.
>
> So this is an interrupt trigger without the interrupt ever
> being visible to software.
>
> It might be easy enough to add that support to the generic version
> except that linking said trigger requires some register changes
> in the STM side. + there is a kicker in the various last bit
> of this patch - we need a way to find out if it's the interrupt
> we think it is (i.e. an exti interrupt)

Hi Jonathan, Linus, all,

First, many thanks for reviewing.

In this patch-set, I choose to implement this hardware trigger line
into separate driver... Thinking out loud:
If I try to summarize, as you perfectly describe here before, I see two 
items to address:
- this is pure HW line, that can either generate interrupts, and/or 
start conversions in HW. This may be hard to combine both, an interrupt 
handler to call iio_trigger_poll() from there, for generic devices, but 
not for stm devices (not sure if this can benefit to others?).
- there is need to do some register changes on stm device side (ADC) as 
well, when choosing a particular trigger (EXTI line for instance) e.g. 
in validate_trigger().

I'm starting to wonder if this can be separate driver. Maybe this
should be part of device driver (e.g. ADC), at least for the time being.

>>
>> Let's see what Jonathan says.
>
>
>>
>>> +config IIO_STM32_EXTI_TRIGGER
>>> +       tristate "STM32 EXTI Trigger"
>>> +       depends on (ARCH_STM32 && OF) || COMPILE_TEST
>>
>> config IIO_GPIO_TRIGGER
>>     depends on GPIOLIB
>>
>>> +       select STM32_EXTI
>>
>> Isn't the dependency actually the other way around?
>>
>> default STM32_EXTI makes more sense, or just put it into the
>> defconfig.
>>
>>> +#include <linux/gpio/consumer.h>
>>> +#include <linux/iio/iio.h>
>>> +#include <linux/iio/trigger.h>
>>> +#include <linux/interrupt.h>
>>> +#include <linux/module.h>
>>> +#include <linux/platform_device.h>
>>> +
>>> +/* STM32 has up to 16 EXTI triggers on GPIOs */
>>> +#define STM32_MAX_EXTI_TRIGGER 16
>>
>> Just don't put any restrictions like this so it can be widely
>> reused.
>>
>>> +static irqreturn_t stm32_exti_trigger_handler(int irq, void *data)
>>> +{
>>> +       /* Exti handler shouldn't be invoked, and isn't used */
>>> +       return IRQ_HANDLED;
>>> +}
>>
>> It could be a good idea to capture the timestamp here if we were
>> actually using this IRQ.
>>
>>> +static int stm32_exti_trigger_probe(struct platform_device *pdev)
>>> +{
>>> +       int irq, ret;
>>> +       char name[8];
>>> +       struct gpio_desc *gpio;
>>> +       struct iio_trigger *trig;
>>> +       unsigned int i;
>>> +
>>> +       for (i = 0; i < STM32_MAX_EXTI_TRIGGER; i++) {
>>
>> Why not just run this until devm_gpiod_get() returns -ERRNO
>> or something?
>>
>>> +               snprintf(name, sizeof(name), "exti%d", i);
>>> +
>>> +               gpio = devm_gpiod_get_optional(&pdev->dev, name, GPIOD_IN);
>>
>> Why would it be optional?
>>
>> Either it is there in the device tree or we get -EINVAL or something
>> if there is no
>> such index in the device tree. We can get -EPROBE_DEEER too, and then
>> we should exit silently or just print that deferring is happening.
>>
>>> +               if (IS_ERR_OR_NULL(gpio)) {
>>> +                       if (IS_ERR(gpio)) {
>>> +                               dev_err(&pdev->dev, "gpio %s get error %ld\n",
>>> +                                       name, PTR_ERR(gpio));
>>> +                               return PTR_ERR(gpio);
>>> +                       }
>>> +                       dev_dbg(&pdev->dev, "No %s gpio\n", name);
>>> +                       continue;
>>> +               }
>>
>> Good
>>
>>> +               irq = gpiod_to_irq(gpio);
>>> +               if (irq < 0) {
>>> +                       dev_err(&pdev->dev, "gpio %d to irq failed\n", i);
>>> +                       return irq;
>>> +               }
>>> +
>>> +               ret = devm_request_irq(&pdev->dev, irq,
>>> +                                      stm32_exti_trigger_handler,
>>> +                                      0, dev_name(&pdev->dev), pdev);
> Hmm. So this is a trick to set the interrupt mapping up inside the device.
> The whole thing doesn't really exist.
>
> Rather feels like there ought to be some generic interface for
> 'I want to pretend I want a particular interrupt but not actually get one'.
>
> But that would only work in this weird case where there is also a real interrupt
> associated with it - just one we elect not to use.
>>> +               if (ret) {
>>> +                       dev_err(&pdev->dev, "request IRQ %d failed\n", irq);
>>> +                       return ret;
>>> +               }
>>
>> Here you need some elaborate trigger edge handling.
>>
>> The flags that you define as "0" here, how do we say that we
>> want to handle rising or falling edges, for example?
>>
>> I think you might want to establish these DT properties for
>> GPIO triggers:
>>
>> gpio-trigger-rising-edge;
>> gpio-trigger-falling-edge;
>>
>> Then:
>>
>> int irq_flags = 0;
>>
>> if (of_property_read_bool(np, "gpio-trigger-rising-edge")
>>    irq_flags |= IRQF_TRIGGER_RISING;
>> else if (of_property_read_bool(np, "gpio-trigger-falling-edge")
>>    irq_flags |= IRQF_TRIGGER_FALLING;
> Should this not all be part of the interrupt specification rather
> than down here in the specific driver?
>>
>> To pass along to the devm_request_irq() function as flags.
>>
>> I find it weird that it even works without. Most GPIO interrupts
>> should require you to set a trigger type. But I guess it is because
>> of the other weirdness you describe below.
>>
>>> +               /*
>>> +                * gpios are configured as interrupts, so exti trigger path is
>>> +                * configured in HW, and can now be used as external trigger
>>> +                * source by other IPs. But getting interrupts when trigger
>>> +                * occurs is unused here, so mask irq on exti controller by
>>> +                * default.
>>> +                */
>>> +               disable_irq(irq);
>>
>> Aha. That is not generic. But what about just adding:
>>
>> if (of_property_read_bool(np, "gpio-trigger-numb-irq")
>>     disable_irq();
>>
>> (Plus add the binding for that something like "this makes the
>> GPIO mentioned get requested, translated to an IRQ, get the
>> IRQ requested, and then immediately just disabled as other
>> hardware will actually hande the IRQ line".)
>>
>> I understand that this is kind of weird: we're making a whole generic
>> GPIO trigger driver just to use it with hardware that grabs and disabled
>> the irq immediately.
>>
>> But I think that in the long run it makes for more reusable code.
> I'd go a step further.  Whether it is numbed or not will depend on what
> is downstream.  We should be providing this interrupt like normal if
> we have other devices triggering off it.  In that case it becomes a standard
> interrupt trigger.

Hmm, in case stm device is using this trigger, iio_trigger_poll() will
be called. If I understand your point here, this interrupt should be 
enabled, when stm device isn't using this trigger (for generic devices).
May validate_device()/set_trigger_state() be used to enable or disable 
this interrupt ? Then, what criteria may be used for that purpose ?

>
> Polling off the back of the dataready interrupt is fine if there is nothing
> earlier available. Here there is so we should really be triggering other
> devices off this earlier interrupt.

I fear it can add complexity, because it will depend on user choice to
select this trigger for an stm32 adc, or another device, or both ?
Not sure how to distinguish both from within this trigger driver.
In the end, best maybe to implement this closer in stm32 adc/dac
driver, and limit trigger usage with .validate_device().

>>
>>> +static const struct of_device_id stm32_exti_trigger_of_match[] = {
>>> +       { .compatible = "st,stm32-exti-trigger" },
>>> +       {},
>>
>> "iio-gpio-trigger"
>>
>> Should fit anyone, given the above amendments.
>>
>>> +#if IS_ENABLED(CONFIG_IIO_STM32_EXTI_TRIGGER)
>>> +bool is_stm32_exti_trigger(struct iio_trigger *trig);
>>> +#else
>>> +static inline bool is_stm32_exti_trigger(struct iio_trigger *trig)
>>> +{
>>> +       return false;
>>> +}
>>> +#endif
>>
>> This seems unnecessary to broadcast to the entire kernel.
> This one section is the only really non generic element that
> isn't supported by the existing interrupt trigger.
> Mind you that doesn't have device tree bindings yet :(

Yes, then I suppose simple approach is to rework this, and put it
inside stm32 adc core driver ?
Then, register trigger from there, and read from dt child node.
Is something like the following suitable ?
adc@...hhh {
	compatible = "st,stm32f4-adc-core"
	...
	adc1 {
		...
	}

	trigger {
		gpios = <...>;
		st,trigger-value = <11>; /* e.g. EXTI nb */
		st,trigger-polarity = <1>;
	}
}

Please let me know your opinion.

Best regards,
Fabrice

>
> I wonder if we can add some sort of flag in there to identify hardware
> blocks for tricks like this. Or at a push provide the interrupt
> to both bits of kit so they can compare and see if they are looking
> at the same one?
>>
>> Why? (Maybe I can find explanations in later patches.
>>
>> Yours,
>> Linus Walleij
>>
>

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