Message-ID: <6e75f5d8-75a0-7948-bb27-ee3c831ddf10@kernel.org>
Date:   Sun, 19 Feb 2017 12:49:39 +0000
From:   Jonathan Cameron <jic23@...nel.org>
To:     Fabrice Gasnier <fabrice.gasnier@...com>,
        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 17/02/17 16:05, Fabrice Gasnier wrote:
> On 02/11/2017 11:29 AM, Jonathan Cameron wrote:
>> On 06/02/17 16:01, Fabrice Gasnier wrote:
>>> 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?).
>> Wouldn't have thought it was that hard to do, but fair enough  + it
>> can definitely be added later if anyone cares.
> 
> Hi Jonathan,
> 
> I hope I'd give some more trials ;-).
> 
>>
>>  - 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.
>> Sure, if it ends up easier that way then do so.  We don't really have
>>  support yet for triggered DACs anyway and as you say this could be
>> modified later - subject to the need to end up with device tree bindings
>> that make sense in that case as well.
> 
> I've just posted an RFC, to add device tree bindings for trigger.
> "iio: trigger: Add OF support and GPIO based trigger"
> I think this is a prerequisite, trying to get more generic implementation.
This is interesting.  Previously in the vast majority of cases we have
left the trigger connectivity up to userspace to define.  The only real
exception has been where we are playing nasty games to do some control and
sampling of a particular analog circuit beyond the ADC.

Will need to think on this and gather opinions on whether it makes sense to
do this.
> 
>>>
>>>>>
>>>>> 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).
>> Exactly - or when both STM and generic drivers are using it, then it
>> would be better to trigger the generic consumer of the trigger at this
>> interrupt, not on the EOC interrupt from the ADC.
> 
> I just have one doubt about this. If I summarize:
> 1 - In case "any device" driver uses a gpio trigger, then, only gpio
> interrupt shall call trigger_poll.
> 2 - In case STM32 ADC device uses gpio trigger, only EOC interrupt shall
> call trigger_poll.
> 3 - In case both 1 + 2, "any device" and STM32 ADC use same trigger...
> Shouldn't trigger_poll() be called only once ? (e.g. not twice ?)
> Then I would rather rely on EOC to call trigger_poll() only once...
> Please advise on this.
You'd drop using the EOC as a trigger at all.  Logically a trigger
is meant to be the event that causes an ADC read to happen.  Now
in some cases the only exposure we have to this is indeed the EOC
type data ready signal, so we hook on to that on the basis it is better
than nothing.  Now if we can get it earlier we are all happy.

In this case, because of the weird and wonderful many options that
exist for triggering the device, it may strangely make sense to have
two triggers. One corresponds to a conventional interrupt trigger (using
the GPIO) and the other to the EOC interrupt for cases when it is being
triggered in a way that isn't 'visible' outside of the hardware module.

Then it would be up to userspace to pick the right one if it wants
to do concurrent sampling.

I'd not worry too much about this though as I think the number of users
actually doing it with any given set of hardware is probably low so
if we don't support doing a particular combination it can be up to the
person who cares to add the support they need!
> 
>>
>>> 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().
>> That is certainly a valid choice.  Not as nice in some ways, but you
>> get to decide on what you work on rather than me ;)
>>
>> I'm not entirely sure how we'd do the magic to identify the trigger
>> and deliberately not hook in the pollfunc if it had nothing to do.
> 
> I hope exposing the GPIO/EXTI trigger -> ADC HW connection to device
> tree can help on this. DT should reflect HW.
> Adding some trigger ops, to inform trigger inhibit the trigger_poll()
> call (and possibly mask the interrupt as well) may do the job ?
Might do (he says without really diving into it ;)
> 
>>>
>>>>>
>>>>>> +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 :(
> 
> Please kindly look at RFC I just sent, and let me know your opinion.
Done so.
> 
>>>
>>> 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.
>> That works for me.  Could conceive of more complex situations that
>> doesn't describe (driving both the ADC and a DAC for example - if
>> that's possible in the hardware).  Can figure out how to do that when it
>> matters.
> 
> I hope I can give some more tries, try to do as generic as possible.
> About your remark on ADC/DAC: Well, I think only possible case is the
> above 1, 2 & 3. DAC isn't there yet. But anyway, it doesn't share same
> lines as ADC in hw :-).
Ah. That makes life easier.
> 
> Best Regards,
> Fabrice
> 
>>
>> Jonathan
>>>
>>> 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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