pwm: sifive: Fix PWM algorithm and clarify inverted compare behavior

The `frac` variable represents the pulse inactive time, and the result
of this algorithm is the pulse active time. Therefore, we must reverse
the result.

Although the SiFive Reference Manual states "pwms >= pwmcmpX -> HIGH",
the hardware behavior is inverted due to a fixed XNOR with 0. As a result,
the pwmcmp register actually defines the low (inactive) portion of the pulse.

The reference is SiFive FU740-C000 Manual[0]

Link: https://sifive.cdn.prismic.io/sifive/1a82e600-1f93-4f41-b2d8-86ed8b16acba_fu740-c000-manual-v1p6.pdf
Co-developed-by: Zong Li <zong.li@sifive.com>
Signed-off-by: Zong Li <zong.li@sifive.com>
Co-developed-by: Vincent Chen <vincent.chen@sifive.com>
Signed-off-by: Vincent Chen <vincent.chen@sifive.com>
Signed-off-by: Nylon Chen <nylon.chen@sifive.com>
Link: https://lore.kernel.org/r/20250529035341.51736-3-nylon.chen@sifive.com
Signed-off-by: Uwe Kleine-König <ukleinek@kernel.org>

diff --git a/drivers/pwm/pwm-sifive.c b/drivers/pwm/pwm-sifive.c
index d5b647e6be7812eb5771941ce3fc40f5f2af9ee2..f3694801d3eed9d03da4824356f8f3a625c00456 100644 (file)
--- 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);