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

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Date:   Tue, 26 Sep 2023 08:16:33 +0100
From:   Sean Young <sean@...s.org>
To:     Ivaylo Dimitrov <ivo.g.dimitrov.75@...il.com>
Cc:     linux-media@...r.kernel.org, Tony Lindgren <tony@...mide.com>,
        Russell King <linux@...linux.org.uk>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Thierry Reding <thierry.reding@...il.com>,
        Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>,
        Timo Kokkonen <timo.t.kokkonen@....fi>,
        Pali Rohár <pali.rohar@...il.com>,
        "Sicelo A . Mhlongo" <absicsz@...il.com>,
        linux-omap@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, linux-pwm@...r.kernel.org
Subject: Re: [PATCH v5 2/2] media: rc: remove ir-rx51 in favour of generic
 pwm-ir-tx

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.

I don't have a n900 to test on, unfortunately.

Thanks
Sean