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pwm: renesas-tpu: Improve maths to compute register settings 

The newly computed register values are intended to exactly match the
previously computed values. The main improvement is that the prescaler
is computed without a loop that involves two divisions in each step.
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@pengutronix.de>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>
This commit is contained in:
Uwe Kleine-König 2022-04-20 14:12:39 +02:00 committed by Thierry Reding
parent 208ab8676b
commit 3c173376ef
1 changed files with 36 additions and 16 deletions
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52
drivers/pwm/pwm-renesas-tpu.c
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@ -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 +253,47 @@ 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);
/*
* Find the minimal prescaler in [0..3] such that
*
* period >> (2 * prescaler) < 0x10000
*
* This could be calculated using something like:
*
* prescaler = max(ilog2(period) / 2, 7) - 7;
*
* but given there are only four allowed results and that ilog2 isn't
* cheap on all platforms using a switch statement is more effective.
*/
switch (period) {
case 1 ... 0xffff:
prescaler = 0;
break;
case 0x10000 ... 0x3ffff:
prescaler = 1;
break;
case 0x40000 ... 0xfffff:
prescaler = 2;
break;
case 0x100000 ... 0x3fffff:
prescaler = 3;
break;
default:
return -EINVAL;
}
if (prescaler == ARRAY_SIZE(prescalers) || period == 0) {
dev_err(&tpu->pdev->dev, "clock rate mismatch\n");
return -ENOTSUPP;
}
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 +303,7 @@ 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;