On Fri, Jul 17, 2020 at 03:37:45PM +0200, Hans de Goede wrote:
> The CRC PWM controller has a clock-divider which divides the clock with
> a value between 1-128. But as can seen from the PWM_DIV_CLK_xxx
> defines, this range maps to a register value of 0-127.
>
> So after calculating the clock-divider we must subtract 1 to get the
> register value, unless the requested frequency was so high that the
> calculation has already resulted in a (rounded) divider value of 0.
>
> Note that before this fix, setting a period of PWM_MAX_PERIOD_NS which
> corresponds to the max. divider value of 128 could have resulted in a
> bug where the code would use 128 as divider-register value which would
> have resulted in an actual divider value of 0 (and the enable bit being
> set). A rounding error stopped this bug from actually happen. This
> same rounding error means that after the subtraction of 1 it is impossible
> to set the divider to 128. Also bump PWM_MAX_PERIOD_NS by 1 ns to allow
> setting a divider of 128 (register-value 127).
Reviewed-by: Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx>
> Signed-off-by: Hans de Goede <hdegoede@xxxxxxxxxx>
> ---
> Changes in v3:
> - Introduce crc_pwm_calc_clk_div() here instead of later in the patch-set
> to reduce the amount of churn in the patch-set a bit
> ---
> drivers/pwm/pwm-crc.c | 17 ++++++++++++++---
> 1 file changed, 14 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/pwm/pwm-crc.c b/drivers/pwm/pwm-crc.c
> index c056eb9b858c..44ec7d5b63e1 100644
> --- a/drivers/pwm/pwm-crc.c
> +++ b/drivers/pwm/pwm-crc.c
> @@ -22,7 +22,7 @@
> #define PWM_MAX_LEVEL 0xFF
>
> #define PWM_BASE_CLK_MHZ 6 /* 6 MHz */
> -#define PWM_MAX_PERIOD_NS 5461333 /* 183 Hz */
> +#define PWM_MAX_PERIOD_NS 5461334 /* 183 Hz */
>
> /**
> * struct crystalcove_pwm - Crystal Cove PWM controller
> @@ -39,6 +39,18 @@ static inline struct crystalcove_pwm *to_crc_pwm(struct pwm_chip *pc)
> return container_of(pc, struct crystalcove_pwm, chip);
> }
>
> +static int crc_pwm_calc_clk_div(int period_ns)
> +{
> + int clk_div;
> +
> + clk_div = PWM_BASE_CLK_MHZ * period_ns / (256 * NSEC_PER_USEC);
> + /* clk_div 1 - 128, maps to register values 0-127 */
> + if (clk_div > 0)
> + clk_div--;
> +
> + return clk_div;
> +}
> +
> static int crc_pwm_enable(struct pwm_chip *c, struct pwm_device *pwm)
> {
> struct crystalcove_pwm *crc_pwm = to_crc_pwm(c);
> @@ -68,11 +80,10 @@ static int crc_pwm_config(struct pwm_chip *c, struct pwm_device *pwm,
> }
>
> if (pwm_get_period(pwm) != period_ns) {
> - int clk_div;
> + int clk_div = crc_pwm_calc_clk_div(period_ns);
>
> /* changing the clk divisor, need to disable fisrt */
> crc_pwm_disable(c, pwm);
> - clk_div = PWM_BASE_CLK_MHZ * period_ns / (256 * NSEC_PER_USEC);
>
> regmap_write(crc_pwm->regmap, PWM0_CLK_DIV,
> clk_div | PWM_OUTPUT_ENABLE);
> --
> 2.26.2
>
--
With Best Regards,
Andy Shevchenko
