@@ -6,7 +6,6 @@
* Limitations:
* - The .apply callback doesn't complete the currently running period before
* reconfiguring the hardware.
- * - Each period starts with the inactive part.
*/
#include <linux/clk.h>
@@ -163,7 +162,7 @@ static int jz4740_pwm_apply(struct pwm_chip *chip, struct pwm_device *pwm,
/* Calculate duty value */
tmp = (unsigned long long)rate * state->duty_cycle;
do_div(tmp, NSEC_PER_SEC);
- duty = period - tmp;
+ duty = tmp;
if (duty >= period)
duty = period - 1;
@@ -189,18 +188,26 @@ static int jz4740_pwm_apply(struct pwm_chip *chip, struct pwm_device *pwm,
regmap_update_bits(jz4740->map, TCU_REG_TCSRc(pwm->hwpwm),
TCU_TCSR_PWM_SD, TCU_TCSR_PWM_SD);
- /* Set polarity */
- switch (state->polarity) {
- case PWM_POLARITY_NORMAL:
+ /*
+ * Set polarity.
+ *
+ * The PWM starts in inactive state until the internal timer reaches the
+ * duty value, then becomes active until the timer reaches the period
+ * value. In theory, we should then use (period - duty) as the real duty
+ * value, as a high duty value would otherwise result in the PWM pin
+ * being inactive most of the time.
+ *
+ * Here, we don't do that, and instead invert the polarity of the PWM
+ * when it is active. This trick makes the PWM start with its active
+ * state instead of its inactive state.
+ */
+ if ((state->polarity == PWM_POLARITY_NORMAL) ^ state->enabled)
regmap_update_bits(jz4740->map, TCU_REG_TCSRc(pwm->hwpwm),
TCU_TCSR_PWM_INITL_HIGH, 0);
- break;
- case PWM_POLARITY_INVERSED:
+ else
regmap_update_bits(jz4740->map, TCU_REG_TCSRc(pwm->hwpwm),
TCU_TCSR_PWM_INITL_HIGH,
TCU_TCSR_PWM_INITL_HIGH);
- break;
- }
if (state->enabled)
jz4740_pwm_enable(chip, pwm);
The PWM in Ingenic SoCs starts in inactive state until the internal timer reaches the duty value, then becomes active until the timer reaches the period value. In theory, we should then use (period - duty) as the real duty value, as a high duty value would otherwise result in the PWM pin being inactive most of the time. This is the reason why the duty value was inverted in the driver until now, but it still had the problem that it would not start with the active part. To address this remaining issue, the common trick is to invert the duty, and invert the polarity when the PWM is enabled. Since the duty was already inverted, and we invert it again, we now program the hardware for the requested duty, and simply invert the polarity when the PWM is enabled. Signed-off-by: Paul Cercueil <paul@crapouillou.net> --- Notes: v2: Add documentation about why we invert the polarity, and improve commit message v3: No change drivers/pwm/pwm-jz4740.c | 25 ++++++++++++++++--------- 1 file changed, 16 insertions(+), 9 deletions(-)