On Wed, Sep 15, 2021 at 08:55:41AM +0200, Wolfram Sang wrote:
> From: Duc Nguyen <duc.nguyen.ub@xxxxxxxxxxx>
>
> We have special code if duty_ns is 0. But if non-zero is given, then the
> calculation should not result in zero duty.
Why not? Assuming a PWM that supports multiples of say 100 ns for
duty_cycle, rounding a request for 550 ns down to 500 ns isn't worse
than rounding down a request for 50 ns to 0 ns is it?
> Signed-off-by: Duc Nguyen <duc.nguyen.ub@xxxxxxxxxxx>
> [wsa: split and reworded commit message]
> Signed-off-by: Wolfram Sang <wsa+renesas@xxxxxxxxxxxxxxxxxxxx>
> ---
> drivers/pwm/pwm-renesas-tpu.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/pwm/pwm-renesas-tpu.c b/drivers/pwm/pwm-renesas-tpu.c
> index 754440194650..bb51156e4bda 100644
> --- a/drivers/pwm/pwm-renesas-tpu.c
> +++ b/drivers/pwm/pwm-renesas-tpu.c
> @@ -275,7 +275,7 @@ static int tpu_pwm_config(struct pwm_chip *chip, struct pwm_device *_pwm,
> if (duty_ns) {
> duty = clk_rate / prescalers[prescaler]
> / (NSEC_PER_SEC / duty_ns);
Unrelated to the change under discussion here: Dividing by the result of
a division is bad.
Consider:
clk_rate = 1333333333
prescalers[prescaler] = 43
duty_ns = 500000001
The exact result is 15503875.996124031, with the above formula you get
31007751 which is off by nearly a factor of two.
These numbers are probably not relevant, but they show the problem.
Best regards
Uwe
--
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