Patch "pwm: tegra: Increase precision in PWM rate calculation" has been added to the 4.4-stable tree

[<gregkh@linuxfoundation.org>]

This is a note to let you know that I've just added the patch titled

    pwm: tegra: Increase precision in PWM rate calculation

to the 4.4-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     pwm-tegra-increase-precision-in-pwm-rate-calculation.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@vger.kernel.org> know about it.


>From foo@baz Mon Mar 19 09:58:12 CET 2018
From: Laxman Dewangan <ldewangan@nvidia.com>
Date: Fri, 7 Apr 2017 15:04:00 +0530
Subject: pwm: tegra: Increase precision in PWM rate calculation

From: Laxman Dewangan <ldewangan@nvidia.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@nvidia.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 drivers/pwm/pwm-tegra.c |    7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

--- a/drivers/pwm/pwm-tegra.c
+++ b/drivers/pwm/pwm-tegra.c
@@ -69,6 +69,7 @@ static int tegra_pwm_config(struct pwm_c
 	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_c
 	 * 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


Patches currently in stable-queue which might be from ldewangan@nvidia.com are

queue-4.4/pwm-tegra-increase-precision-in-pwm-rate-calculation.patch