From e5ca42458b6278b7d5866e08dae7c45349af2157 Mon Sep 17 00:00:00 2001
From: Dan O'Donovan <dan@emutex.com>
Date: Wed, 1 Jun 2016 15:31:12 +0100
Subject: pwm: lpss: Fix base_unit calculation for PWM frequency

The base_unit calculation applies an offset of 0x2 which adds
significant error for lower frequencies and doesn't appear to be
warranted - rounding the division result gives a correct value.

Also, the upper limit check for base_unit is off-by-one; the upper
nibble of base_unit is invalid if >=128 according to the Table 88
in the Z8000 Processor Series Datasheet Volume 1 (Rev. 2).

Verified on UP Board (Cherry Trail) and Minnowboard Max (Bay Trail).

Signed-off-by: Dan O'Donovan <dan@emutex.com>
Acked-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>
---
 drivers/pwm/pwm-lpss.c | 8 +++-----
 1 file changed, 3 insertions(+), 5 deletions(-)

(limited to 'drivers/pwm')

diff --git a/drivers/pwm/pwm-lpss.c b/drivers/pwm/pwm-lpss.c
index 295b963dbddb..98dc8b80b79d 100644
--- a/drivers/pwm/pwm-lpss.c
+++ b/drivers/pwm/pwm-lpss.c
@@ -27,7 +27,6 @@
 #define PWM_SW_UPDATE			BIT(30)
 #define PWM_BASE_UNIT_SHIFT		8
 #define PWM_ON_TIME_DIV_MASK		0x000000ff
-#define PWM_DIVISION_CORRECTION		0x2
 
 /* Size of each PWM register space if multiple */
 #define PWM_SIZE			0x400
@@ -101,17 +100,16 @@ static int pwm_lpss_config(struct pwm_chip *chip, struct pwm_device *pwm,
 
 	/*
 	 * The equation is:
-	 * base_unit = ((freq / c) * base_unit_range) + correction
+	 * base_unit = round(base_unit_range * freq / c)
 	 */
 	base_unit_range = BIT(lpwm->info->base_unit_bits);
-	base_unit = freq * base_unit_range;
+	freq *= base_unit_range;
 
 	c = lpwm->info->clk_rate;
 	if (!c)
 		return -EINVAL;
 
-	do_div(base_unit, c);
-	base_unit += PWM_DIVISION_CORRECTION;
+	base_unit = DIV_ROUND_CLOSEST_ULL(freq, c);
 
 	if (duty_ns <= 0)
 		duty_ns = 1;
-- 
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