diff options
| author | David Rivshin <drivshin@allworx.com> | 2016-01-29 23:26:51 -0500 |
|---|---|---|
| committer | Thierry Reding <thierry.reding@gmail.com> | 2016-03-23 17:11:45 +0100 |
| commit | f8caa792261c0edded20eba2b8fcc899a1b91819 (patch) | |
| tree | 7920bbf9a010fe99d53ac154f7fc20253127f3f3 | |
| parent | c5857e3f94ab2719dfac649a146cb5dd6f21fcf3 (diff) | |
| download | lwn-f8caa792261c0edded20eba2b8fcc899a1b91819.tar.gz lwn-f8caa792261c0edded20eba2b8fcc899a1b91819.zip | |
pwm: omap-dmtimer: Fix inaccurate period and duty cycle calculations
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@allworx.com>
Acked-by: Neil Armstrong <narmstrong@baylibre.com>
Tested-by: Adam Ford <aford173@gmail.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>
| -rw-r--r-- | drivers/pwm/pwm-omap-dmtimer.c | 27 |
1 files changed, 20 insertions, 7 deletions
diff --git a/drivers/pwm/pwm-omap-dmtimer.c b/drivers/pwm/pwm-omap-dmtimer.c index 826634ec0d5c..0083e75d950f 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 |