Re: [PATCH 3/3] input: gpio-keys: Use hrtimer for software debounce

On Sun, Mar 07, 2021 at 09:11:18PM +0000, Paul Cercueil wrote:

Hi Dmitry,

Le dim. 7 mars 2021 à 12:20, Dmitry Torokhov [off-list ref] a
écrit :

quoted

On Fri, Mar 05, 2021 at 08:00:43PM +0000, Paul Cercueil wrote:

quoted

 Hi Dmitry,

 Le ven. 5 mars 2021 à 10:35, Dmitry Torokhov
[off-list ref] a
 écrit :
 > Hi Paul,
 >
 > On Fri, Mar 05, 2021 at 05:01:11PM +0000, Paul Cercueil wrote:
 > >  -static void gpio_keys_gpio_work_func(struct work_struct *work)
 > >  +static enum hrtimer_restart gpio_keys_debounce_timer(struct
 > > hrtimer *t)
 > >   {
 > >  -	struct gpio_button_data *bdata =
 > >  -		container_of(work, struct gpio_button_data, work.work);
 > >  +	struct gpio_button_data *bdata = container_of(t,
 > >  +						      struct gpio_button_data,
 > >  +						      debounce_timer);
 > >
 > >   	gpio_keys_gpio_report_event(bdata);
 >
 > I am not sure how this works. As far as I know, even
 > HRTIMER_MODE_REL_SOFT do not allow sleeping in the timer
handlers, and
 > gpio_keys_gpio_report_event() use sleeping variant of GPIOD API
(and
 > that is not going to change).

 Quoting <linux/hrtimers.h>, the "timer callback will be executed in
soft irq
 context", so sleeping should be possible.

I am afraid you misunderstand what soft irq context is, as softirqs and
tasklets still run in interrupt context and therefore can not sleep,
only code running in process context may sleep.

I probably do. My understanding of "softirq" is that the callback runs in a
threaded interrupt handler.

No, you are thinking about threaded interrupts, which are separate
beasts. Softirqs are traditional bottom halfs that run after exit of
hard interrupt, but still are non-preemptible so sleeping is not
allowed.

quoted

You can test it yourself by sticking "msleep(1)" in
gpio_keys_debounce_timer() and see if you will get "scheduling while
atomic" in logs.

I tested it, it locks up.

quoted

quoted

 But I guess in this case I can use HRTIMER_MODE_REL.

This changes selected clock source, but has no effect on whether timer
handler can sleep or not.

quoted

 > It seems to me that if you want to use software debounce in gpio
keys
 > driver you need to set up sufficiently high HZ for your system.
Maybe we
 > could thrown a warning when we see low debounce delay and low HZ
to
 > alert system developer.

 This is exactly what we should not do. I certainly don't want to
have 250+
 timer interrupts per second just so that input events aren't lost,
to work
 around a sucky debounce implementation. Besides, if you consider the
 hrtimers doc (Documentation/timers/hrtimers.rst), hrtimers really
are what
 should be used here.

I explained why they can't. They could be if you restrict gpio_keys to
only be used with GPIOs that do not require sleep to read their state,
but I am not willing to accept such restriction. You either need to have
longer debounce, higher HZ, or see if you can use GPIO controller that
supports debounce handling. See also if you can enable dynamic
ticks/NO_HZ to limit number of timer interrupts on idle system.

We can also use the hrtimer approach if the GPIO doesn't require sleep, and
fall back to the standard timer if it does. It's possible to detect that
with gpiod_cansleep(). The diff would be pretty slim. Would you accept
something like that?

Switching from HZ=250 to HZ=24 leads to a 3% overall performance increase
across all apps on our system, so a pretty big optimization, and this is the
only blocker.

Let me take a look at the updated patch and we will see.

Thanks.

-- 
Dmitry