Re: [PATCH v2 4/5] iio: trigger: add support for STM32 EXTI triggers

From: Fabrice Gasnier
Date: Mon Feb 06 2017 - 11:03:34 EST

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@xxxxxx> 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@xxxxxx>

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@0xhhhh {
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