Commit 3c173376 authored by Uwe Kleine-König's avatar Uwe Kleine-König Committed by Thierry Reding
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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: default avatarUwe Kleine-König <u.kleine-koenig@pengutronix.de>
Reviewed-by: default avatarGeert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: default avatarThierry Reding <thierry.reding@gmail.com>
parent 208ab867

drivers/pwm/pwm-renesas-tpu.c

+35 −15
Original line number Diff line number Diff line
@@ -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;



	clk_rate = clk_get_rate(tpu->clk);



	period = clk_rate / (NSEC_PER_SEC / period_ns);



	/*

	 * Pick a prescaler to avoid overflowing the counter.

	 * TODO: Pick the highest acceptable prescaler.

	 * 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.

	 */

	clk_rate = clk_get_rate(tpu->clk);

	switch (period) {

	case 1 ... 0xffff:

		prescaler = 0;

		break;



	for (prescaler = 0; prescaler < ARRAY_SIZE(prescalers); ++prescaler) {

		period = clk_rate / prescalers[prescaler]

		       / (NSEC_PER_SEC / period_ns);

		if (period <= 0xffff)

	case 0x10000 ... 0x3ffff:

		prescaler = 1;

		break;



	case 0x40000 ... 0xfffff:

		prescaler = 2;

		break;

	}



	if (prescaler == ARRAY_SIZE(prescalers) || period == 0) {

		dev_err(&tpu->pdev->dev, "clock rate mismatch\n");

		return -ENOTSUPP;

	case 0x100000 ... 0x3fffff:

		prescaler = 3;

		break;



	default:

		return -EINVAL;

	}



	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;