linux-kernel - Re: [PATCH 3/3] pwm-sun4i: calculate the delay without rounding down to jiffies

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Message-ID: <20220303215640.m23yozioeqxvmlfc@pengutronix.de>
Date:   Thu, 3 Mar 2022 22:56:40 +0100
From:   Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
To:     Max Kellermann <max.kellermann@...il.com>
Cc:     linux-pwm@...r.kernel.org, thierry.reding@...il.com,
        lee.jones@...aro.org, linux-kernel@...r.kernel.org,
        andrey@...edev.lt, stable@...r.kernel.org
Subject: Re: [PATCH 3/3] pwm-sun4i: calculate the delay without rounding down
 to jiffies

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@...r.kernel.org
> > Signed-off-by: Max Kellermann <max.kellermann@...il.com>
> 
> Sounds reasonable
> 
> Acked-by; Uwe Kleine-König <u.kleine-koenig@...gutronix.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@...gutronix.de>

Thanks
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | https://www.pengutronix.de/ |

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