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

From: Tony Lindgren <tony@atomide.com>
To: Sean Young <sean@mess.org>
Cc: "Ivaylo Dimitrov" <ivo.g.dimitrov.75@gmail.com>,
	linux-media@vger.kernel.org,
	"Russell King" <linux@armlinux.org.uk>,
	"Mauro Carvalho Chehab" <mchehab@kernel.org>,
	"Thierry Reding" <thierry.reding@gmail.com>,
	"Uwe Kleine-König" <u.kleine-koenig@pengutronix.de>,
	"Timo Kokkonen" <timo.t.kokkonen@iki.fi>,
	"Pali Rohár" <pali.rohar@gmail.com>,
	"Sicelo A . Mhlongo" <absicsz@gmail.com>,
	linux-omap@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org, linux-pwm@vger.kernel.org
Subject: Re: [PATCH v5 2/2] media: rc: remove ir-rx51 in favour of generic pwm-ir-tx
Date: Tue, 26 Sep 2023 10:37:44 +0300	[thread overview]
Message-ID: <20230926073744.GA5285@atomide.com> (raw)
In-Reply-To: <ZRKFUb1vRtn82bgn@gofer.mess.org>

* Sean Young <sean@mess.org> [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.

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.

> 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.

Regards,

Tony