iio: hrtimer-trigger: Mark hrtimer to expire in hard interrupt context

On PREEMPT_RT enabled kernels unmarked hrtimers are moved into soft
interrupt expiry mode by default.

The IIO hrtimer-trigger needs to run in hard interrupt context since it
will end up calling generic_handle_irq() which has the requirement to run
in hard interrupt context.

Explicitly specify that the timer needs to run in hard interrupt context by
using the HRTIMER_MODE_REL_HARD flag.

Fixes: f5c2f0215e36 ("hrtimer: Move unmarked hrtimers to soft interrupt expiry on RT")
Reported-by: Christian Eggers <[e-mail]>
Signed-off-by: Lars-Peter Clausen <[e-mail]>
Acked-by: Sebastian Andrzej Siewior <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jonathan Cameron <[e-mail]>

drivers/iio/trigger/iio-trig-hrtimer.c

···
 
 	if (state)
 		hrtimer_start(&trig_info->timer, trig_info->period,
-			      HRTIMER_MODE_REL);
+			      HRTIMER_MODE_REL_HARD);
 	else
 		hrtimer_cancel(&trig_info->timer);
 
···
 	trig_info->swt.trigger->ops = &iio_hrtimer_trigger_ops;
 	trig_info->swt.trigger->dev.groups = iio_hrtimer_attr_groups;
 
-	hrtimer_init(&trig_info->timer, CLOCK_MONOTONIC, HRTIMER_MODE_REL);
+	hrtimer_init(&trig_info->timer, CLOCK_MONOTONIC, HRTIMER_MODE_REL_HARD);
 	trig_info->timer.function = iio_hrtimer_trig_handler;
 
 	trig_info->sampling_frequency = HRTIMER_DEFAULT_SAMPLING_FREQUENCY;