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

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Date:   Tue, 25 Jan 2022 13:34:29 +0100
From:   Max Kellermann <max.kellermann@...il.com>
To:     linux-pwm@...r.kernel.org, thierry.reding@...il.com,
        u.kleine-koenig@...gutronix.de, lee.jones@...aro.org
Cc:     linux-kernel@...r.kernel.org, andrey@...edev.lt,
        Max Kellermann <max.kellermann@...il.com>,
        stable@...r.kernel.org
Subject: [PATCH 3/3] pwm-sun4i: calculate the delay without rounding down to jiffies

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>
---
 drivers/pwm/pwm-sun4i.c | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/drivers/pwm/pwm-sun4i.c b/drivers/pwm/pwm-sun4i.c
index b44deececb8b..8527a655034c 100644
--- a/drivers/pwm/pwm-sun4i.c
+++ b/drivers/pwm/pwm-sun4i.c
@@ -234,7 +234,6 @@ static int sun4i_pwm_apply(struct pwm_chip *chip, struct pwm_device *pwm,
 	struct pwm_state cstate;
 	u32 ctrl, duty = 0, period = 0, val;
 	int ret;
-	unsigned int delay_jiffies;
 	unsigned int delay_us, prescaler = 0;
 	bool bypass;

@@ -302,8 +301,7 @@ static int sun4i_pwm_apply(struct pwm_chip *chip, struct pwm_device *pwm,
 		return 0;

 	/* We need a full period to elapse before disabling the channel. */
-	delay_jiffies = nsecs_to_jiffies(cstate.period + 1000);
-	delay_us = jiffies_to_usecs(delay_jiffies);
+	delay_us = DIV_ROUND_UP_ULL(cstate.period, NSEC_PER_USEC);
 	if ((delay_us / 500) > MAX_UDELAY_MS)
 		msleep(delay_us / 1000 + 1);
 	else
-- 
2.34.0