diff options
| author | Uwe Kleine-König <u.kleine-koenig@pengutronix.de> | 2020-12-21 17:19:13 +0100 |
|---|---|---|
| committer | Uwe Kleine-König <uwe@kleine-koenig.org> | 2021-01-12 19:57:37 +0100 |
| commit | b8fc6fa1defbf6d3a179193bea38349c527f07dc (patch) | |
| tree | cbd544faef6b10e4493951c13f9c7afd2b55d838 | |
| parent | fba28bc569f7033a315ab10f1a07a6954a58e73c (diff) | |
| download | linux-pwm-round-state.tar.gz linux-pwm-round-state.tar.xz | |
pwm: bcm2835: Improve period and duty cycle calculationpwm-round-state
With an input clk rate bigger than 2000000000, scaler would have been
zero which then would have resulted in a division by zero.
Also the originally implemented algorithm divided by the result of a
division. This nearly always looses precision. Consider a requested period
of 1000000 ns. With an input clock frequency of 32786885 Hz the hardware
was configured with an actual period of 983869.007 ns (PERIOD = 32258)
while the hardware can provide 1000003.508 ns (PERIOD = 32787).
And note if the input clock frequency was 32786886 Hz instead, the hardware
was configured to 1016656.477 ns (PERIOD = 33333) while the optimal
setting results in 1000003.477 ns (PERIOD = 32787).
This patch implements proper range checking and only divides once for
the calculation of period (and similar for duty_cycle).
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
| -rw-r--r-- | drivers/pwm/pwm-bcm2835.c | 35 |
1 files changed, 27 insertions, 8 deletions
diff --git a/drivers/pwm/pwm-bcm2835.c b/drivers/pwm/pwm-bcm2835.c index 6ff5f04b3e07..d593cce249d9 100644 --- a/drivers/pwm/pwm-bcm2835.c +++ b/drivers/pwm/pwm-bcm2835.c @@ -64,8 +64,9 @@ static int bcm2835_pwm_apply(struct pwm_chip *chip, struct pwm_device *pwm, struct bcm2835_pwm *pc = to_bcm2835_pwm(chip); unsigned long rate = clk_get_rate(pc->clk); - unsigned long long period; - unsigned long scaler; + unsigned long long period_cycles; + u64 max_period; + u32 val; if (!rate) { @@ -73,18 +74,36 @@ static int bcm2835_pwm_apply(struct pwm_chip *chip, struct pwm_device *pwm, return -EINVAL; } - scaler = DIV_ROUND_CLOSEST(NSEC_PER_SEC, rate); + /* + * period_cycles must be a 32 bit value, so period * rate / NSEC_PER_SEC + * must be <= U32_MAX. As U32_MAX * NSEC_PER_SEC < U64_MAX the + * multiplication period * rate doesn't overflow. + * To calculate the maximal possible period that guarantees the + * above inequality: + * + * round(period * rate / NSEC_PER_SEC) <= U32_MAX + * <=> period * rate / NSEC_PER_SEC < U32_MAX + 0.5 + * <=> period * rate < (U32_MAX + 0.5) * NSEC_PER_SEC + * <=> period < ((U32_MAX + 0.5) * NSEC_PER_SEC) / rate + * <=> period < ((U32_MAX * NSEC_PER_SEC + NSEC_PER_SEC/2) / rate + * <=> period <= ceil((U32_MAX * NSEC_PER_SEC + NSEC_PER_SEC/2) / rate) - 1 + */ + max_period = DIV_ROUND_UP_ULL((u64)U32_MAX * NSEC_PER_SEC + NSEC_PER_SEC / 2, rate) - 1; + + if (state->period > max_period) + return -EINVAL; + /* set period */ - period = DIV_ROUND_CLOSEST_ULL(state->period, scaler); + period_cycles = DIV_ROUND_CLOSEST_ULL(state->period * rate, NSEC_PER_SEC); - /* dont accept a period that is too small or has been truncated */ - if ((period < PERIOD_MIN) || (period > U32_MAX)) + /* don't accept a period that is too small */ + if (period_cycles < PERIOD_MIN) return -EINVAL; - writel(period, pc->base + PERIOD(pwm->hwpwm)); + writel(period_cycles, pc->base + PERIOD(pwm->hwpwm)); /* set duty cycle */ - val = DIV_ROUND_CLOSEST_ULL(state->duty_cycle, scaler); + val = DIV_ROUND_CLOSEST_ULL(state->duty_cycle * rate, NSEC_PER_SEC); writel(val, pc->base + DUTY(pwm->hwpwm)); /* set polarity */ |