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

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Message-ID: <20220224165135.g4ufknd3alrhnfx3@pengutronix.de>
Date:   Thu, 24 Feb 2022 17:51:35 +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 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>

Best regards
Uwe

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

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