Re: overflow and wrong timeout errors in pwm-atmel

From: Alexandre Belloni <alexandre.belloni@bootlin.com>
To: "Uwe Kleine-König" <u.kleine-koenig@pengutronix.de>
Cc: Claudiu Beznea <claudiu.beznea@microchip.com>,
	Thierry Reding <thierry.reding@gmail.com>,
	Lee Jones <lee.jones@linaro.org>,
	linux-pwm@vger.kernel.org,
	Nicolas Ferre <nicolas.ferre@microchip.com>,
	Ludovic Desroches <ludovic.desroches@microchip.com>,
	kernel@pengutronix.de, 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