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

[Sean Young]

On Tue, Sep 26, 2023 at 03:43:18PM +0300, Ivaylo Dimitrov wrote:
> On 26.09.23 г. 10:16 ч., Sean Young wrote:
> > 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.
> > 
> 
> Could you elaborate on the "pwm-ir-tx driver does suffer from this problem"?
> Where do you see that?

So on a raspberry pi (model 3b), if I use the pwm-ir-tx driver, I get random
delays of up to 100us. It's a bit random and certainly depends on the load.

I'm measuring using a logic analyzer.

There have been reports by others on different machines with random delays
and/or transmit failures (as in the receiver occassionally fails to decode
the IR). I usually suggest they use the gpio-ir-tx driver, which does work
as far as I know (the signal looks perfect with a logic analyzer).

So far I've taken the view that the driver works ok for most situations,
since IR is usually fine with upto 100us missing here or there.

The gpio-ir-tx driver works much better because it does the entire send 
under spinlock - obviously that has its own problems, because an IR transmit
can be 10s or even 100s of milliseconds.

I've never known of a solution to the pwm-ir-tx driver. If using hrtimers
directly improves the situation even a bit, then that would be great.

> ir-rx51 does not suffer from the same problem (albeit it has its own one,
> see bellow)
> 
> > > 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.
> > 
> 
> usleep_range() is backed by hrtimers already, however the difference comes
> from how hrtimer is used in ir-rx51: it uses timer callback function that
> gets called in softirq context, while usleep_range() puts the task in
> TASK_UNINTERRUPTIBLE state and then calls schedule_hrtimeout_range(). For
> some reason it takes at least 200-400 us (on average) even on OMAP4 to
> switch back to TASK_RUNNING state.
> 
> The issue with ir-rx51 and the way it uses hrtimers is that it calls
> pwm_apply_state() from hrtimer function, which is not ok, per the comment
> here
> https://elixir.bootlin.com/linux/v6.6-rc3/source/drivers/pwm/core.c#L502
> 
> I can make pwm-ir-tx switch to hrtimers, that's not an issue, but I am
> afraid that there is some general scheduler or timers (or something else)
> issue that manifests itself with usleep_range() misbehaving.

If we can switch pwm-ir-tx to hrtimers, that would be great.

The ir-rx51 removal patches have already been queued to media_staging;
we may have to remove them from there if we can't solve this problem.

> > I don't have a n900 to test on, unfortunately.
> > 
> 
> I have and once I have an idea what's going on will port pwm-ir-tx to
> hrtimers, if needed. Don't want to do it now as I am afraid the completion I
> will have to use will have the same latency problems as usleep_range()

That would be fantastic. Please do keep us up to date with how you are
getting on. Like I said, it would be nice to this resolved before the next
merge window.

Thanks,
Sean