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authorUwe Kleine-König <u.kleine-koenig@pengutronix.de>2021-01-14 21:48:04 +0100
committerThierry Reding <thierry.reding@gmail.com>2021-03-22 11:48:07 +0100
commitca0d2fb790eb26fc53d851007ed1ead6c048be11 (patch)
treee80ce9c3b4fda243f785b559b358a8d3a5af4281 /drivers/pwm/pwm-bcm2835.c
parenta38fd8748464831584a19438cbb3082b5a2dab15 (diff)
pwm: bcm2835: Improve period and duty cycle calculation
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> Reviewed-by: Lino Sanfilippo <LinoSanfilippo@gmx.de> Tested-by: Lino Sanfilippo <LinoSanfilippo@gmx.de> Signed-off-by: Thierry Reding <thierry.reding@gmail.com>
Diffstat (limited to 'drivers/pwm/pwm-bcm2835.c')
-rw-r--r--drivers/pwm/pwm-bcm2835.c35
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 */