[PATCH 1/4] pwm: omap-dmtimer: fix inaccurate period/duty_cycle calculation

[Neil Armstrong]

On 01/30/2016 05:26 AM, David Rivshin (Allworx) wrote:
> From: David Rivshin <drivshin at allworx.com>
> 
> Fix the calculation of load_value and match_value. Currently they
> are slightly too low, which produces a noticeably wrong PWM rate with
> sufficiently short periods (i.e. when 1/period approaches clk_rate/2).
> 
> Example:
>  clk_rate=32768Hz, period=122070ns, duty_cycle=61035ns (8192Hz/50% PWM)
>  Correct values: load = 0xfffffffc, match = 0xfffffffd
>  Current values: load = 0xfffffffa, match = 0xfffffffc
>  effective PWM: period=183105ns, duty_cycle=91553ns (5461Hz/50% PWM)
> 
> Fixes: 6604c6556db9 ("pwm: Add PWM driver for OMAP using dual-mode timers")
> Signed-off-by: David Rivshin <drivshin at allworx.com>

OK for me.

Acked-by: Neil Armstrong <narmstrong at baylibre.com>

Thanks !

> ---
>  drivers/pwm/pwm-omap-dmtimer.c | 27 ++++++++++++++++++++-------
>  1 file changed, 20 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/pwm/pwm-omap-dmtimer.c b/drivers/pwm/pwm-omap-dmtimer.c
> index 826634e..0083e75 100644
> --- a/drivers/pwm/pwm-omap-dmtimer.c
> +++ b/drivers/pwm/pwm-omap-dmtimer.c
> @@ -31,6 +31,7 @@
>  #include <linux/time.h>
>  
>  #define DM_TIMER_LOAD_MIN 0xfffffffe
> +#define DM_TIMER_MAX      0xffffffff
>  
>  struct pwm_omap_dmtimer_chip {
>  	struct pwm_chip chip;
> @@ -46,13 +47,13 @@ to_pwm_omap_dmtimer_chip(struct pwm_chip *chip)
>  	return container_of(chip, struct pwm_omap_dmtimer_chip, chip);
>  }
>  
> -static int pwm_omap_dmtimer_calc_value(unsigned long clk_rate, int ns)
> +static u32 pwm_omap_dmtimer_get_clock_cycles(unsigned long clk_rate, int ns)
>  {
>  	u64 c = (u64)clk_rate * ns;
>  
>  	do_div(c, NSEC_PER_SEC);
>  
> -	return DM_TIMER_LOAD_MIN - c;
> +	return c;
>  }
>  
>  static void pwm_omap_dmtimer_start(struct pwm_omap_dmtimer_chip *omap)
> @@ -99,7 +100,8 @@ static int pwm_omap_dmtimer_config(struct pwm_chip *chip,
>  				   int duty_ns, int period_ns)
>  {
>  	struct pwm_omap_dmtimer_chip *omap = to_pwm_omap_dmtimer_chip(chip);
> -	int load_value, match_value;
> +	u32 period_cycles, duty_cycles;
> +	u32 load_value, match_value;
>  	struct clk *fclk;
>  	unsigned long clk_rate;
>  	bool timer_active;
> @@ -133,11 +135,22 @@ static int pwm_omap_dmtimer_config(struct pwm_chip *chip,
>  	/*
>  	 * Calculate the appropriate load and match values based on the
>  	 * specified period and duty cycle. The load value determines the
> -	 * cycle time and the match value determines the duty cycle.
> +	 * period time and the match value determines the duty time.
> +	 *
> +	 * The period lasts for (DM_TIMER_MAX-load_value+1) clock cycles.
> +	 * Similarly, the active time lasts (match_value-load_value+1) cycles.
> +	 * The non-active time is the remainder: (DM_TIMER_MAX-match_value)
> +	 * clock cycles.
> +	 *
> +	 * References:
> +	 *   OMAP4430/60/70 TRM sections 22.2.4.10 and 22.2.4.11
> +	 *   AM335x Sitara TRM sections 20.1.3.5 and 20.1.3.6
>  	 */
> -	load_value = pwm_omap_dmtimer_calc_value(clk_rate, period_ns);
> -	match_value = pwm_omap_dmtimer_calc_value(clk_rate,
> -						  period_ns - duty_ns);
> +	period_cycles = pwm_omap_dmtimer_get_clock_cycles(clk_rate, period_ns);
> +	duty_cycles = pwm_omap_dmtimer_get_clock_cycles(clk_rate, duty_ns);
> +
> +	load_value = (DM_TIMER_MAX - period_cycles) + 1;
> +	match_value = load_value + duty_cycles - 1;
>  
>  	/*
>  	 * We MUST stop the associated dual-mode timer before attempting to
>