[Intel-gfx] [PATCH v5 08/16] pwm: crc: Fix off-by-one error in the clock-divider calculations

[Andy Shevchenko]

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 at linux.intel.com>

> Signed-off-by: Hans de Goede <hdegoede at redhat.com>
> ---
> 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