From: "Uwe Kleine-König" <u.kleine-koenig@pengutronix.de>
To: Wolfram Sang <wsa+renesas@sang-engineering.com>
Cc: linux-pwm@vger.kernel.org, linux-renesas-soc@vger.kernel.org,
Duc Nguyen <duc.nguyen.ub@renesas.com>,
Thierry Reding <thierry.reding@gmail.com>,
Lee Jones <lee.jones@linaro.org>,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH 2/2] pwm: renesas-tpu: don't allow no duty if duty_ns is given
Date: Thu, 16 Sep 2021 18:47:46 +0200 [thread overview]
Message-ID: <20210916164746.rujqwpnrm4i6lf7z@pengutronix.de> (raw)
In-Reply-To: <20210915065542.1897-3-wsa+renesas@sang-engineering.com>
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On Wed, Sep 15, 2021 at 08:55:41AM +0200, Wolfram Sang wrote:
> From: Duc Nguyen <duc.nguyen.ub@renesas.com>
>
> 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@renesas.com>
> [wsa: split and reworded commit message]
> Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
> ---
> 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
--
Pengutronix e.K. | Uwe Kleine-König |
Industrial Linux Solutions | https://www.pengutronix.de/ |
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prev parent reply other threads:[~2021-09-16 18:30 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-09-15 6:55 [PATCH 0/2] pwm: renesas-tpu: minor improvements Wolfram Sang
2021-09-15 6:55 ` [PATCH 1/2] pwm: renesas-tpu: better errno for impossible rates Wolfram Sang
2021-09-15 9:36 ` Sergei Shtylyov
2021-09-17 8:25 ` Uwe Kleine-König
2021-09-20 9:08 ` Wolfram Sang
2021-09-15 6:55 ` [PATCH 2/2] pwm: renesas-tpu: don't allow no duty if duty_ns is given Wolfram Sang
2021-09-16 16:47 ` Uwe Kleine-König [this message]
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