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

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Date:   Fri, 4 Dec 2020 08:44:17 +0000
From:   Sean Young <sean@...s.org>
To:     Lino Sanfilippo <LinoSanfilippo@....de>
Cc:     Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>, 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
Subject: Re: [PATCH v2] pwm: bcm2835: Support apply function for atomic
 configuration

Hi,

On Fri, Dec 04, 2020 at 12:42:15AM +0100, Lino Sanfilippo 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;

Rather than doing another 64 bit division which is expensive (esp on 32 bit
kernels), you could assign to u64 and check:

	if (value < PERIOD || value > U32_MAX)
		return -EINVAL;

> > Also note that round_closed is probably wrong, as .apply() is
> > supposed to round down the period to the next achievable period. (But
> > fixing this has to do done in a separate patch.)
> 
> According to commit 11fc4edc4 rounding to the closest integer has been introduced
> to improve precision in case that the pwm controller is used by the pwm-ir-tx driver.
> I dont know how strong the requirement is to round down the period in apply(), but I
> can imagine that this may be a good reason to deviate from this rule.
> (CCing Sean Young who introduced DIV_ROUND_CLOSEST)

There was a problem where the carrier is incorrect for some IR hardware
which uses a carrier of 455kHz. With periods that small, rounding errors
do really matter and rounding down might cause problems.

A policy of rounding down the carrier is not the right thing to do
for pwm-ir-tx, and such a change will probably break pwm-ir-tx in some
edge cases.

Thanks

Sean