[PATCH AUTOSEL 6.6 28/59] pwm: sifive: Fix PWM algorithm and clarify inverted compare behavior

[Sasha Levin <sashal@kernel.org>]

From: Nylon Chen <nylon.chen@sifive.com>

[ Upstream commit 7dbc4432ea6bf9d709391eb57f1e9fb44e99845a ]

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 [0]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---

LLM Generated explanations, may be completely bogus:

**Backport Status: YES**

This commit should be backported to stable kernel trees. Here's my
extensive analysis:

## Critical Bug Fix Analysis

1. **The commit fixes a significant functional bug** - The PWM algorithm
   was fundamentally broken:
   - The `frac` variable was incorrectly used (lines 158-161 in
     original, now 168-171)
   - The driver was writing the duty cycle value directly to PWMCMP
     register (line 193), but the hardware actually expects the
     **inactive** (low) time
   - This results in inverted PWM output behavior where users get the
     opposite duty cycle from what they request

2. **Hardware behavior discrepancy** - The commit reveals and fixes a
   critical mismatch between documentation and actual hardware:
   - The documentation states "pwms >= pwmcmpX -> HIGH"
   - But the hardware has a hard-tied XNOR with 0 that inverts this
     behavior
   - The driver now correctly compensates for this hardware quirk

3. **User-visible impact**:
   - **In `pwm_sifive_get_state()`**: The driver now correctly inverts
     the value read from hardware (lines 115->122-126)
   - **In `pwm_sifive_apply()`**: The driver now inverts the duty cycle
     before writing to hardware (lines 162->171)
   - **Polarity change**: Changed from `PWM_POLARITY_INVERSED` to
     `PWM_POLARITY_NORMAL` (lines 126->137, 142->152)
   - **Documentation fix**: Updated comment from "cannot generate 100%
     duty" to "cannot generate 0% duty" (lines 11->27, 160->170)

4. **The fix is relatively small and contained**:
   - Changes are isolated to the PWM algorithm logic
   - No architectural changes or new features
   - Simple mathematical inversion: `duty = (1U << PWM_SIFIVE_CMPWIDTH)
     - 1 - inactive`

5. **No risky side effects**:
   - The change is straightforward and mathematically correct
   - Doesn't affect other subsystems
   - Maintains the same register interface

6. **Affects all SiFive PWM users**:
   - Any system using SiFive FU540 or FU740 SoCs would have incorrect
     PWM output
   - This includes various RISC-V development boards and embedded
     systems
   - Users would get inverted duty cycles, potentially breaking motor
     controls, LED dimming, etc.

7. **Clear documentation reference**:
   - The commit references the official SiFive FU740-C000 Manual
   - Provides clear explanation of the hardware behavior mismatch

The bug causes PWM outputs to be inverted from user expectations, which
is a significant functional issue that would affect any system relying
on proper PWM behavior for motor control, LED dimming, or other PWM-
dependent functionality. The fix is clean, minimal, and addresses a
clear hardware/software mismatch that exists in production systems.

 drivers/pwm/pwm-sifive.c | 39 +++++++++++++++++++++++++++++++--------
 1 file changed, 31 insertions(+), 8 deletions(-)

diff --git a/drivers/pwm/pwm-sifive.c b/drivers/pwm/pwm-sifive.c
index eabddb7c7820..2e6253c671df 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);
-- 
2.39.5