linux-kernel - [PATCH AUTOSEL for 4.4 009/101] pwm: tegra: Increase precision in PWM rate calculation

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Date:   Thu, 8 Mar 2018 05:01:12 +0000
From:   Sasha Levin <Alexander.Levin@...rosoft.com>
To:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>
CC:     Laxman Dewangan <ldewangan@...dia.com>,
        Thierry Reding <thierry.reding@...il.com>,
        Sasha Levin <Alexander.Levin@...rosoft.com>
Subject: [PATCH AUTOSEL for 4.4 009/101] pwm: tegra: Increase precision in PWM
 rate calculation

From: Laxman Dewangan <ldewangan@...dia.com>

[ Upstream commit 250b76f43f57d578ebff5e7211eb2c73aa5cd6ca ]

The rate of the PWM calculated as follows:

	hz = NSEC_PER_SEC / period_ns;
 	rate = (rate + (hz / 2)) / hz;

This has the precision loss in lower PWM rate.

Change this to have more precision as:

	hz = DIV_ROUND_CLOSEST_ULL(NSEC_PER_SEC * 100, period_ns);
	rate = DIV_ROUND_CLOSEST(rate * 100, hz)

Example:

1. period_ns = 16672000, PWM clock rate is 200 KHz.

	Based on old formula
		hz = NSEC_PER_SEC / period_ns
		   = 1000000000ul/16672000
		   = 59 (59.98)
		rate = (200K + 59/2)/59 = 3390

	Based on new method:
		hz = 5998
		rate = DIV_ROUND_CLOSE(200000*100, 5998) = 3334

	If we measure the PWM signal rate, we will get more accurate
	period with rate value of 3334 instead of 3390.

2.  period_ns = 16803898, PWM clock rate is 200 KHz.

	Based on old formula:
		hz = 59, rate = 3390

	Based on new formula:
		hz = 5951, rate = 3360

	The PWM signal rate of 3360 is more near to requested period
	than 3333.

Signed-off-by: Laxman Dewangan <ldewangan@...dia.com>
Signed-off-by: Thierry Reding <thierry.reding@...il.com>
Signed-off-by: Sasha Levin <alexander.levin@...rosoft.com>
---
 drivers/pwm/pwm-tegra.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/drivers/pwm/pwm-tegra.c b/drivers/pwm/pwm-tegra.c
index d4de0607b502..3039fb762893 100644
--- a/drivers/pwm/pwm-tegra.c
+++ b/drivers/pwm/pwm-tegra.c
@@ -69,6 +69,7 @@ static int tegra_pwm_config(struct pwm_chip *chip, struct pwm_device *pwm,
 	struct tegra_pwm_chip *pc = to_tegra_pwm_chip(chip);
 	unsigned long long c;
 	unsigned long rate, hz;
+	unsigned long long ns100 = NSEC_PER_SEC;
 	u32 val = 0;
 	int err;
 
@@ -87,9 +88,11 @@ static int tegra_pwm_config(struct pwm_chip *chip, struct pwm_device *pwm,
 	 * cycles at the PWM clock rate will take period_ns nanoseconds.
 	 */
 	rate = clk_get_rate(pc->clk) >> PWM_DUTY_WIDTH;
-	hz = NSEC_PER_SEC / period_ns;
 
-	rate = (rate + (hz / 2)) / hz;
+	/* Consider precision in PWM_SCALE_WIDTH rate calculation */
+	ns100 *= 100;
+	hz = DIV_ROUND_CLOSEST_ULL(ns100, period_ns);
+	rate = DIV_ROUND_CLOSEST(rate * 100, hz);
 
 	/*
 	 * Since the actual PWM divider is the register's frequency divider
-- 
2.14.1