[PATCH v11 14/27] pwm: jz4740: Improve algorithm of clock calculation

[Paul Cercueil]

The previous algorithm hardcoded details about how the TCU clocks work.

The new algorithm will compute the maximum frequency at which the clock
can run for the given period/duty parameters, and use clk_set_max_rate()
to ensure that we will always compute period/duty values that fit in our
16-bit register fields.

Signed-off-by: Paul Cercueil <paul@xxxxxxxxxxxxxxx>
Tested-by: Mathieu Malaterre <malat@xxxxxxxxxx>
Tested-by: Artur Rojek <contact@xxxxxxxxxxxxxx>
---
 drivers/pwm/pwm-jz4740.c | 51 +++++++++++++++++++++++++++++++++---------------
 1 file changed, 35 insertions(+), 16 deletions(-)

diff --git a/drivers/pwm/pwm-jz4740.c b/drivers/pwm/pwm-jz4740.c
index 1a0eab0abafe..b55ee879e23d 100644
--- a/drivers/pwm/pwm-jz4740.c
+++ b/drivers/pwm/pwm-jz4740.c
@@ -108,24 +108,47 @@ static int jz4740_pwm_apply(struct pwm_chip *chip, struct pwm_device *pwm,
 	struct jz4740_pwm_chip *jz4740 = to_jz4740(pwm->chip);
 	struct clk *clk = pwm_get_chip_data(pwm),
 		   *parent_clk = clk_get_parent(clk);
-	unsigned long rate, period, duty;
+	unsigned long rate, parent_rate, period, duty;
 	unsigned long long tmp;
-	unsigned int prescaler = 0;
+	int ret;
 
-	rate = clk_get_rate(parent_clk);
-	tmp = (unsigned long long)rate * state->period;
-	do_div(tmp, 1000000000);
-	period = tmp;
+	parent_rate = clk_get_rate(parent_clk);
+
+	jz4740_pwm_disable(chip, pwm);
+
+	/* Reset the clock to the maximum rate, and we'll reduce it if needed */
+	ret = clk_set_rate(clk, parent_rate);
+	if (ret)
+		return ret;
 
-	while (period > 0xffff && prescaler < 6) {
-		period >>= 2;
-		rate >>= 2;
-		++prescaler;
+	/*
+	 * Limit the clock to a maximum rate that still gives us a period value
+	 * which fits in 16 bits.
+	 */
+	tmp = 0xffffull * NSEC_PER_SEC;
+	do_div(tmp, state->period);
+
+	ret = clk_set_max_rate(clk, tmp);
+	if (ret) {
+		dev_err(chip->dev, "Unable to set max rate: %i\n", ret);
+		return ret;
 	}
 
-	if (prescaler == 6)
-		return -EINVAL;
+	/*
+	 * Read back the clock rate, as it may have been modified by
+	 * clk_set_max_rate()
+	 */
+	rate = clk_get_rate(clk);
+
+	if (rate != parent_rate)
+		dev_dbg(chip->dev, "PWM clock updated to %lu Hz\n", rate);
 
+	/* Calculate period value */
+	tmp = (unsigned long long)rate * state->period;
+	do_div(tmp, NSEC_PER_SEC);
+	period = (unsigned long)tmp;
+
+	/* Calculate duty value */
 	tmp = (unsigned long long)period * state->duty_cycle;
 	do_div(tmp, state->period);
 	duty = period - tmp;
@@ -133,8 +156,6 @@ static int jz4740_pwm_apply(struct pwm_chip *chip, struct pwm_device *pwm,
 	if (duty >= period)
 		duty = period - 1;
 
-	jz4740_pwm_disable(chip, pwm);
-
 	/*
 	 * Set abrupt shutdown.
 	 *
@@ -144,8 +165,6 @@ 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);
 
-	clk_set_rate(clk, rate);
-
 	/* Reset counter to 0 */
 	regmap_write(jz4740->map, TCU_REG_TCNTc(pwm->hwpwm), 0);
 
-- 
2.11.0