From: "Uwe Kleine-König" <u.kleine-koenig@pengutronix.de>
To: Max Kellermann <max.kellermann@gmail.com>
Cc: linux-pwm@vger.kernel.org, thierry.reding@gmail.com,
lee.jones@linaro.org, linux-kernel@vger.kernel.org,
andrey@lebedev.lt, stable@vger.kernel.org
Subject: Re: [PATCH 3/3] pwm-sun4i: calculate the delay without rounding down to jiffies
Date: Thu, 3 Mar 2022 22:56:40 +0100 [thread overview]
Message-ID: <20220303215640.m23yozioeqxvmlfc@pengutronix.de> (raw)
In-Reply-To: <20220224165135.g4ufknd3alrhnfx3@pengutronix.de>
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On Thu, Feb 24, 2022 at 05:51:35PM +0100, Uwe Kleine-König wrote:
> On Tue, Jan 25, 2022 at 01:34:29PM +0100, Max Kellermann wrote:
> > This fixes a problem that was supposed to be addressed by commit
> > 6eefb79d6f5bc ("pwm: sun4i: Remove erroneous else branch") - backlight
> > could not be switched off on some Allwinner A20. The commit was
> > correct, but was not a reliable fix for the problem, which was timing
> > related.
> >
> > The real problem for the backlight switching problem was that sleeping
> > for a full period did not work, because delay_us is always zero.
> >
> > It is zero because the period (plus 1 microsecond) is rounded down to
> > the next "jiffies", but the period is less than one jiffy.
> >
> > On my Cubieboard 2, the period is 5ms, and 1 jiffy (at the default
> > HZ=100) is 10ms, so nsecs_to_jiffies(10ms+1us)=0.
> >
> > The roundtrip from nanoseconds to jiffies and back to microseconds is
> > an unnecessary loss of precision; always rounding down (via
> > nsecs_to_jiffies()) then causes the breakage.
> >
> > This patch eliminates this roundtrip, and directly converts from
> > nanoseconds to microseconds (for usleep_range()), using
> > DIV_ROUND_UP_ULL() to force rounding up. This way, the sleep time is
> > never zero, and after the sleep, we are guaranteed to be in a
> > different period, and the device is ready for another control command
> > for sure.
> >
> > Cc: stable@vger.kernel.org
> > Signed-off-by: Max Kellermann <max.kellermann@gmail.com>
>
> Sounds reasonable
>
> Acked-by; Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
/me had problems with is keyboard, the Shift-key didn't work and so I
typed a ; instead a :. To make patchwork pick up my tag, I'll repeat it
here:
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Thanks
Uwe
--
Pengutronix e.K. | Uwe Kleine-König |
Industrial Linux Solutions | https://www.pengutronix.de/ |
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next prev parent reply other threads:[~2022-03-03 21:56 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-01-25 12:34 [PATCH 1/3] pwm-sun4i: convert "next_period" to local variable Max Kellermann
2022-01-25 12:34 ` [PATCH 2/3] pwm-sun4i: calculate "delay_jiffies" directly, eliminate absolute time Max Kellermann
2022-02-24 16:50 ` Uwe Kleine-König
2022-01-25 12:34 ` [PATCH 3/3] pwm-sun4i: calculate the delay without rounding down to jiffies Max Kellermann
2022-02-24 16:51 ` Uwe Kleine-König
2022-03-03 21:56 ` Uwe Kleine-König [this message]
2022-01-25 14:31 ` [PATCH 1/3] pwm-sun4i: convert "next_period" to local variable Uwe Kleine-König
2022-01-25 14:39 ` Max Kellermann
2022-01-25 14:43 ` Uwe Kleine-König
2022-02-24 13:04 ` Thierry Reding
2022-02-24 16:49 ` Uwe Kleine-König
2022-04-22 15:47 ` Thierry Reding
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