[PATCH 5/6] pwm: renesas-tpu: Improve maths to compute register settings

[Uwe Kleine-König <u.kleine-koenig@xxxxxxxxxxxxxx>]

The newly computed register values are intended to exactly match the
previously computed values. The main improvement is that the prescaler
is computed directly instead of with a loop. This uses the fact, that
prescalers[i] = 1 << (2 * i).

Assuming a moderately smart compiler, the needed number of divisions for
the case where the requested period is too big, is reduced from 5 to 2.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@xxxxxxxxxxxxxx>
---
 drivers/pwm/pwm-renesas-tpu.c | 28 +++++++++++-----------------
 1 file changed, 11 insertions(+), 17 deletions(-)

diff --git a/drivers/pwm/pwm-renesas-tpu.c b/drivers/pwm/pwm-renesas-tpu.c
index 671f1f824da8..fce7df418d62 100644
--- a/drivers/pwm/pwm-renesas-tpu.c
+++ b/drivers/pwm/pwm-renesas-tpu.c
@@ -244,7 +244,6 @@ static void tpu_pwm_free(struct pwm_chip *chip, struct pwm_device *pwm)
 static int tpu_pwm_config(struct pwm_chip *chip, struct pwm_device *pwm,
 			  int duty_ns, int period_ns, bool enabled)
 {
-	static const unsigned int prescalers[] = { 1, 4, 16, 64 };
 	struct tpu_pwm_device *tpd = pwm_get_chip_data(pwm);
 	struct tpu_device *tpu = to_tpu_device(chip);
 	unsigned int prescaler;
@@ -254,26 +253,21 @@ static int tpu_pwm_config(struct pwm_chip *chip, struct pwm_device *pwm,
 	u32 duty;
 	int ret;
 
-	/*
-	 * Pick a prescaler to avoid overflowing the counter.
-	 * TODO: Pick the highest acceptable prescaler.
-	 */
 	clk_rate = clk_get_rate(tpu->clk);
 
-	for (prescaler = 0; prescaler < ARRAY_SIZE(prescalers); ++prescaler) {
-		period = clk_rate / prescalers[prescaler]
-		       / (NSEC_PER_SEC / period_ns);
-		if (period <= 0xffff)
-			break;
-	}
+	period = clk_rate / (NSEC_PER_SEC / period_ns);
+	if (period >= 64 * 0x10000 || period == 0)
+		return -EINVAL;
 
-	if (prescaler == ARRAY_SIZE(prescalers) || period == 0) {
-		dev_err(&tpu->pdev->dev, "clock rate mismatch\n");
-		return -ENOTSUPP;
-	}
+	if (period < 0x10000)
+		prescaler = 0;
+	else
+		prescaler = ilog2(period / 0x10000) / 2 + 1;
+
+	period >>= 2 * prescaler;
 
 	if (duty_ns) {
-		duty = clk_rate / prescalers[prescaler]
+		duty = (clk_rate >> 2 * prescaler)
 		     / (NSEC_PER_SEC / duty_ns);
 		if (duty > period)
 			return -EINVAL;
@@ -283,7 +277,7 @@ static int tpu_pwm_config(struct pwm_chip *chip, struct pwm_device *pwm,
 
 	dev_dbg(&tpu->pdev->dev,
 		"rate %u, prescaler %u, period %u, duty %u\n",
-		clk_rate, prescalers[prescaler], period, duty);
+		clk_rate, 1 << (2 * prescaler), period, duty);
 
 	if (tpd->prescaler == prescaler && tpd->period == period)
 		duty_only = true;
-- 
2.35.1