Re: [PATCH 1/2] iio: hrtimer-trigger: Mark hrtimer to expire in hard interrupt context

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

On 2020-08-13 09:53:57 [+0200], Lars-Peter Clausen wrote:
> 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.

No, I don't think that this is good. It basically renders threaded-irqs
in context of IIO useless. This also requires that the IRQ-handler in
question runs with IRQs disabled / uses raw_spinlock_t which is in not
good idea either.

Has this change (including the second patch in thread) been tested on RT
in terms of locking and latency?

> Fixes: f5c2f0215e36 ("hrtimer: Move unmarked hrtimers to soft interrupt expiry on RT")
> Reported-by: Christian Eggers <ceggers@arri.de>
> Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
> ---
>  drivers/iio/trigger/iio-trig-hrtimer.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/iio/trigger/iio-trig-hrtimer.c b/drivers/iio/trigger/iio-trig-hrtimer.c
> index f59bf8d58586..b11ca915fcb2 100644
> --- a/drivers/iio/trigger/iio-trig-hrtimer.c
> +++ b/drivers/iio/trigger/iio-trig-hrtimer.c
> @@ -132,7 +132,7 @@ static struct iio_sw_trigger *iio_trig_hrtimer_probe(const char *name)
>  	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;

Sebastian