From: Alexandre Belloni <alexandre.belloni@bootlin.com>
To: "Uwe Kleine-König" <u.kleine-koenig@pengutronix.de>
Cc: linux-pwm@vger.kernel.org,
Ludovic Desroches <ludovic.desroches@microchip.com>,
Thierry Reding <thierry.reding@gmail.com>,
kernel@pengutronix.de, Lee Jones <lee.jones@linaro.org>,
Claudiu Beznea <claudiu.beznea@microchip.com>,
linux-arm-kernel@lists.infradead.org
Subject: Re: overflow and wrong timeout errors in pwm-atmel
Date: Wed, 21 Apr 2021 16:18:37 +0200 [thread overview]
Message-ID: <YIA0PXR+oxxpkrzL@piout.net> (raw)
In-Reply-To: <20210421134825.powx5tvqvce32fho@pengutronix.de>
On 21/04/2021 15:48:25+0200, Uwe Kleine-König wrote:
> On Wed, Apr 21, 2021 at 01:03:36PM +0200, Uwe Kleine-König wrote:
> > On Wed, Apr 21, 2021 at 11:26:08AM +0200, Uwe Kleine-König wrote:
> > > With these three patches PWM_DEBUG is now happy. (At least I couldn't
> > > trigger a warning any more. I think there are still a few problems with
> > > integer overflows.)
> >
> > BTW, setting the period to 138350580899 (with a clock rate of 133333333
> > Hz) results in setting period=0 because
> >
> > state->period * clkrate =
> > 138350580899 * 133333333 =
> > 40254751 (discarded from 18446744073749806367).
>
> As a first remedy the following could be done:
>
> diff --git a/drivers/pwm/pwm-atmel.c b/drivers/pwm/pwm-atmel.c
> index 38d86340201c..02d69fa5f7d2 100644
> --- a/drivers/pwm/pwm-atmel.c
> +++ b/drivers/pwm/pwm-atmel.c
> @@ -199,6 +199,11 @@ static int atmel_pwm_calculate_cprd_and_pres(struct pwm_chip *chip,
> unsigned long long cycles = state->period;
> int shift;
>
> + if (fls(cycles) + fls(clkrate) > 64) {
> + dev_err(chip->dev, "period to big to calculate HW parameters\n");
> + return -EINVAL;
> + }
> +
> /* Calculate the period cycles and prescale value */
> cycles *= clkrate;
> do_div(cycles, NSEC_PER_SEC);
>
> Is this sensible? (Actually I'd prefer to just continue with
>
> period = (ULL(1) << (64 - fls(clkrate))) - 1
>
> according to the motto to yield the highest possible period, but this
> function has another error path that returns -EINVAL so this would be
> inconsistent.)
Shouldn't that be -ERANGE? I do think it is better to return an error
and let userspace decide what is the policy instead of having the policy
in the driver.
--
Alexandre Belloni, co-owner and COO, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com
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next prev parent reply other threads:[~2021-04-21 14:20 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-04-20 9:51 [PATCH 1/2] pwm: atmel: Fix duty cycle calculation in .get_state() Uwe Kleine-König
2021-04-20 9:51 ` [PATCH 2/2] pwm: atmel: Improve duty cycle calculation in .apply() Uwe Kleine-König
2021-04-23 17:07 ` Thierry Reding
2021-04-21 9:26 ` [PATCH] pwm: atmel: rework tracking updates pending in hardware Uwe Kleine-König
[not found] ` <20210421110336.bd5s6e2kjxqilddi@pengutronix.de>
2021-04-21 13:48 ` overflow and wrong timeout errors in pwm-atmel Uwe Kleine-König
2021-04-21 14:18 ` Alexandre Belloni [this message]
2021-04-21 15:26 ` Uwe Kleine-König
2021-07-05 7:55 ` [PATCH] pwm: atmel: rework tracking updates pending in hardware Uwe Kleine-König
2021-04-23 17:07 ` [PATCH 1/2] pwm: atmel: Fix duty cycle calculation in .get_state() Thierry Reding
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