Re: [PATCH v5 2/2] media: rc: remove ir-rx51 in favour of generic pwm-ir-tx

[Ivaylo Dimitrov]

On 26.09.23 г. 10:37 ч., Tony Lindgren wrote:
> * Sean Young <sean@xxxxxxxx> [230926 07:16]:
> > On Mon, Sep 25, 2023 at 07:06:44PM +0300, Ivaylo Dimitrov wrote:
> > > On 1.09.23 г. 17:18 ч., Sean Young wrote:
> > > > The ir-rx51 is a pwm-based TX driver specific to the N900. This can be
> > > > handled entirely by the generic pwm-ir-tx driver, and in fact the
> > > > pwm-ir-tx driver has been compatible with ir-rx51 from the start.
> > >
> > > Unfortunately, pwm-ir-tx does not work on n900. My investigation shows that
> > > for some reason usleep_range() sleeps for at least 300-400 us more than what
> > > interval it is requested to sleep. I played with cyclictest from rt-tests
> > > package and it gives similar results - increasing the priority helps, but I
> > > was not able to make it sleep for less that 300 us in average. I tried
> > > cpu_latency_qos_add_request() in pwm-ir-tx, but it made no difference.
> > >
> > > I get similar results on motorola droid4 (OMAP4), albeit there average sleep
> > > is in 200-300 us range, which makes me believe that either OMAPs have issues
> > > with hrtimers or the config we use has some issue which leads to scheduler
> > > latency. Or, something else...
> >
> > The pwm-ir-tx driver does suffer from this problem, but I was under the
> > impression that the ir-rx51 has the same problem.
> >
> > > In either case help is appreciated to dig further trying to find the reason
> > > for such a big delay.
> >
> > pwm-ir-tx uses usleep_range() and ir-rx51 uses hrtimers. I thought that
> > usleep_range() uses hrtimers; however if you're not seeing the same delay
> > on ir-rx51 then maybe it's time to switch pwm-ir-tx to hrtimers.
>
> Maybe using fsleep() fixes this issue? See commit c6af13d33475 ("timer: add
> fsleep for flexible sleeping"), and Documentation/timers/timers-howto.rst.

I doubt, time intervals we are talking about are > 500 us, which means 
fsleep will always use usleep_range() (or even worse, msleep()), see 
https://elixir.bootlin.com/linux/v6.6-rc3/source/include/linux/delay.h#L82

> The long wake-up time for an idle state could explain the values. I think
> Ivaylo already tested with most cpuidle states disabled via sysfs though.

Yes, I disabled all idle states on both n900 and droid4 (when doing 
cyclictest experiments), with no difference. I also locked frequency on 
n900 to 500MHz, which improved the things a bit, by some 20-50 us 
(IIRC), which makes sense, but also makes me think frequency scaling is 
not the one to blame either.

> > I don't have a n900 to test on, unfortunately.
>
> If you want one for development, the maemo folks cc:ed here likely have
> some available devices.

I think we can arrange one, yes, but my gut feeling tells me the issue 
is not n900 specific, it is just a bit worse there as the device is 
relatively slow already. I have no sane explanation why one would see 
similar latencies on droid4, given that it is times faster than n900.

Regards,

Ivo

> Regards,
>
> Tony