@@ -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>
@@ -113,6 +130,10 @@ static int pwm_sifive_get_state(struct pwm_chip *chip, struct pwm_device *pwm,
u32 duty, val, inactive;
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;