From 3c173376efc461dc670b02ba2846c2a533491104 Mon Sep 17 00:00:00 2001 From: =?utf8?q?Uwe=20Kleine-K=C3=B6nig?= Date: Wed, 20 Apr 2022 14:12:39 +0200 Subject: [PATCH] pwm: renesas-tpu: Improve maths to compute register settings MIME-Version: 1.0 Content-Type: text/plain; charset=utf8 Content-Transfer-Encoding: 8bit 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 Reviewed-by: Geert Uytterhoeven Signed-off-by: Thierry Reding --- drivers/pwm/pwm-renesas-tpu.c | 50 ++++++++++++++++++++++++++++++------------- 1 file changed, 35 insertions(+), 15 deletions(-) diff --git a/drivers/pwm/pwm-renesas-tpu.c b/drivers/pwm/pwm-renesas-tpu.c index 5bced20..4aff3870 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 +253,47 @@ 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) - break; - } + case 0x10000 ... 0x3ffff: + prescaler = 1; + break; - if (prescaler == ARRAY_SIZE(prescalers) || period == 0) { - dev_err(&tpu->pdev->dev, "clock rate mismatch\n"); - return -ENOTSUPP; + case 0x40000 ... 0xfffff: + prescaler = 2; + break; + + 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; -- 2.7.4