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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>
hifive-unleashed-5.1
Dan O'Donovan 2016-06-01 15:31:12 +01:00 committed by Thierry Reding
parent cc37655e6b
commit e5ca42458b
1 changed files with 3 additions and 5 deletions
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8
drivers/pwm/pwm-lpss.c
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@ -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;