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>

1 files changed, 3 insertions, 5 deletions

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;