Re: [PATCH v10 10/10] iio: accel: mma8452: Support interrupt sharing

[Esben Haabendal]

"Andy Shevchenko" <andriy.shevchenko@xxxxxxxxx> writes:

> On Mon, Sep 28, 2026 at 10:26:26AM +0200, Esben Haabendal wrote:
>> Adding support for sharing interrupt line with other device requires the
>> interrupt handler to handle runtime PM suspension properly, ignoring the
>> irq if the device is suspended (maybe even off). And while at it, we use
>> the PM reference to ensure we do not get suspended while processing an irq.
>>
>> In order to prevent the chip from raising irq while suspended (that is when
>> using fixed regulator, where suspend just means setting the device in
>> STANDBY mode), we disable all interrupt sources by clearing CTRL_REG4, and
>> then restores the value again when resuming.
>>
>> The scoped_guard in mma8452_runtime_suspend() is changed to a plain
>> mutex_lock() instead, both to prevent mixing guards and goto, but also to
>> ensure that we stay in STANDBY mode while writing to CTRL_REG4 and all the
>> way up to disabling the device as much as possible.
>>
>> With that in place, it is safe to add the IRQF_SHARED flag.
>>
>> Keep in mind that the device by default is using push-pull for the irq pin,
>> which might require additional hardware design to allow interrupt sharing.
>
> ...
>
>> +	pm_status = pm_runtime_get_if_active(dev);
>> +	if (pm_status == 0)
>> +		/* device is powered down */
>> +		return IRQ_NONE;
>
>> +	if (IS_ENABLED(CONFIG_PM) && pm_status < 0)
>> +		/* runtime PM was disabled, possibly suspending */
>> +		return IRQ_HANDLED;
>
> This is very interesting part. I bet this will be the first driver using this.
> A big question "why?"

Because as I understood it, I needed to add this for the patch to be
accepted [1]. It is definitely possible that I have misunderstood that.

The technical reason I believe is that during system suspend,
pm_runtime_force_suspend() disabled runtime PM, and if a shared
interrupt durings this, pm_runtime_get_if_active returns -EINVAL, and we
could therefore end up doing I2C reads on unpowered hardware.

Per that reasoning, we probably should be doing this in other drivers as
well.

>> +	/*
>> +	 * pm_status is now 1 or -EINVAL (with CONFIG_PM not enabled). If
>> +	 * pm_status==1, runtime PM is enabled and device is RPM_ACTIVE. If
>> +	 * pm_status==-EINVAL, runtime PM is build-time disabled (i.e. CONFIG_PM
>> +	 * not enabled), and we can/must assume device is active.
>> +	 */
>> +
>>  	src = i2c_smbus_read_byte_data(data->client, MMA8452_INT_SRC);
>>  	if (src < 0)
>> -		return IRQ_NONE;
>> +		goto out_runtime_put;
>>
>>  	if (!(src & (data->chip_info->enabled_events | MMA8452_INT_DRDY)))
>> -		return IRQ_NONE;
>> +		goto out_runtime_put;
>>
>>  	if (src & MMA8452_INT_DRDY) {
>>  		iio_trigger_poll_nested(indio_dev->trig);
>> @@ -1120,6 +1139,10 @@ static irqreturn_t mma8452_interrupt(int irq, void *p)
>>  		ret = IRQ_HANDLED;
>>  	}
>>
>> +out_runtime_put:
>> +	if (pm_status > 0)
>> +		pm_runtime_put_autosuspend(dev);
>> +
>>  	return ret;
>>  }
>
> ...
>
>> +	pm_runtime_enable(dev);
>> +	pm_runtime_set_autosuspend_delay(dev, MMA8452_AUTO_SUSPEND_DELAY_MS);
>> +	pm_runtime_use_autosuspend(dev);
>
> I don't see the respective _dont_use_autosuspend() call anywhere.

Ah. I was not aware it was attached to a usage counter as well. I will
add a pm_runtime_dont_use_autosuspend() to the runtime_suspend: error handling.

> ...
>
> I am wondering how many of the above possible scenarios you were able
> to test.

Not enough. I haven't done any additional instrumentation to cause all
the various error conditions that is handled here in _probe().

> ...
>
> P.S.  I bet you can now make a presentation "PM runtime in Linux and
> why it is so hard."

LOL. I am getting there, yes. I have definitely learned a lot about how
runtime PM works. Especially since I was mostly starting from scratch on
that.

But i fear it would easily end up being quite a chaotic presentation :)

/Esben