diff options
Diffstat (limited to 'drivers/pwm/pwm-sifive.c')
| -rw-r--r-- | drivers/pwm/pwm-sifive.c | 39 |
1 files changed, 31 insertions, 8 deletions
diff --git a/drivers/pwm/pwm-sifive.c b/drivers/pwm/pwm-sifive.c index d5b647e6be78..f3694801d3ee 100644 --- a/drivers/pwm/pwm-sifive.c +++ b/drivers/pwm/pwm-sifive.c @@ -4,11 +4,28 @@ * For SiFive's PWM IP block documentation please refer Chapter 14 of * Reference Manual : https://static.dev.sifive.com/FU540-C000-v1.0.pdf * + * PWM output inversion: According to the SiFive Reference manual + * the output of each comparator is high whenever the value of pwms is + * greater than or equal to the corresponding pwmcmpX[Reference Manual]. + * + * Figure 29 in the same manual shows that the pwmcmpXcenter bit is + * hard-tied to 0 (XNOR), which effectively inverts the comparison so that + * the output goes HIGH when `pwms < pwmcmpX`. + * + * In other words, each pwmcmp register actually defines the **inactive** + * (low) period of the pulse, not the active time exactly opposite to what + * the documentation text implies. + * + * To compensate, this driver always **inverts** the duty value when reading + * or writing pwmcmp registers , so that users interact with a conventional + * **active-high** PWM interface. + * + * * Limitations: * - When changing both duty cycle and period, we cannot prevent in * software that the output might produce a period with mixed * settings (new period length and old duty cycle). - * - The hardware cannot generate a 100% duty cycle. + * - The hardware cannot generate a 0% duty cycle. * - The hardware generates only inverted output. */ #include <linux/clk.h> @@ -110,9 +127,14 @@ static int pwm_sifive_get_state(struct pwm_chip *chip, struct pwm_device *pwm, struct pwm_state *state) { struct pwm_sifive_ddata *ddata = pwm_sifive_chip_to_ddata(chip); - u32 duty, val; + u32 duty, val, inactive; - duty = readl(ddata->regs + PWM_SIFIVE_PWMCMP(pwm->hwpwm)); + inactive = readl(ddata->regs + PWM_SIFIVE_PWMCMP(pwm->hwpwm)); + /* + * PWM hardware uses 'inactive' counts in pwmcmp, so invert to get actual duty. + * Here, 'inactive' is the low time and we compute duty as max_count - inactive. + */ + duty = (1U << PWM_SIFIVE_CMPWIDTH) - 1 - inactive; state->enabled = duty > 0; @@ -123,7 +145,7 @@ static int pwm_sifive_get_state(struct pwm_chip *chip, struct pwm_device *pwm, state->period = ddata->real_period; state->duty_cycle = (u64)duty * ddata->real_period >> PWM_SIFIVE_CMPWIDTH; - state->polarity = PWM_POLARITY_INVERSED; + state->polarity = PWM_POLARITY_NORMAL; return 0; } @@ -137,9 +159,9 @@ static int pwm_sifive_apply(struct pwm_chip *chip, struct pwm_device *pwm, unsigned long long num; bool enabled; int ret = 0; - u32 frac; + u32 frac, inactive; - if (state->polarity != PWM_POLARITY_INVERSED) + if (state->polarity != PWM_POLARITY_NORMAL) return -EINVAL; cur_state = pwm->state; @@ -157,8 +179,9 @@ static int pwm_sifive_apply(struct pwm_chip *chip, struct pwm_device *pwm, */ num = (u64)duty_cycle * (1U << PWM_SIFIVE_CMPWIDTH); frac = DIV64_U64_ROUND_CLOSEST(num, state->period); - /* The hardware cannot generate a 100% duty cycle */ + /* The hardware cannot generate a 0% duty cycle */ frac = min(frac, (1U << PWM_SIFIVE_CMPWIDTH) - 1); + inactive = (1U << PWM_SIFIVE_CMPWIDTH) - 1 - frac; mutex_lock(&ddata->lock); if (state->period != ddata->approx_period) { @@ -190,7 +213,7 @@ static int pwm_sifive_apply(struct pwm_chip *chip, struct pwm_device *pwm, } } - writel(frac, ddata->regs + PWM_SIFIVE_PWMCMP(pwm->hwpwm)); + writel(inactive, ddata->regs + PWM_SIFIVE_PWMCMP(pwm->hwpwm)); if (!state->enabled) clk_disable(ddata->clk); |