linux-kernel - Re: [PATCH v2] pwm: bcm2835: Support apply function for atomic configuration

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Message-ID: <20201204112115.wopx5p5elgte7gad@pengutronix.de>
Date:   Fri, 4 Dec 2020 12:21:15 +0100
From:   Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
To:     Lino Sanfilippo <LinoSanfilippo@....de>
Cc:     thierry.reding@...il.com, lee.jones@...aro.org,
        nsaenzjulienne@...e.de, f.fainelli@...il.com, rjui@...adcom.com,
        sbranden@...adcom.com, bcm-kernel-feedback-list@...adcom.com,
        linux-pwm@...r.kernel.org, linux-rpi-kernel@...ts.infradead.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        sean@...s.org
Subject: Re: [PATCH v2] pwm: bcm2835: Support apply function for atomic
 configuration

Hello Lino,

On Fri, Dec 04, 2020 at 12:42:15AM +0100, Lino Sanfilippo wrote:
> On 29.11.20 at 19:10, Uwe Kleine-König wrote:
> > You're storing an unsigned long long (i.e. 64 bits) in an u32. If
> > you are sure that this won't discard relevant bits, please explain
> > this in a comment for the cursory reader.
> 
> What about an extra check then to make sure that the period has not been truncated,
> e.g:
> 
> 	value = DIV_ROUND_CLOSEST_ULL(state->period, scaler);
> 
> 	/* dont accept a period that is too small or has been truncated */
> 	if ((value < PERIOD_MIN) ||
> 	    (value != DIV_ROUND_CLOSEST_ULL(state->period, scaler)))
> 		return -EINVAL;

I'd make value an unsigned long long and check for > 0xffffffff instead
of repeating the (expensive) division. (Hmm, maybe the compiler is smart
enough to not actually repeat it, but still.)

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | https://www.pengutronix.de/ |

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