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

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Date:   Fri, 4 Dec 2020 08:58:39 +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

On Fri, Dec 04, 2020 at 08:44:17AM +0000, Sean Young wrote:
> On Fri, Dec 04, 2020 at 12:42:15AM +0100, Lino Sanfilippo wrote:
> > 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.

Let me rephrase that.

Changing the division to rounding down will exactly revert the fix I made
in commit 11fc4edc483bea8bf0efa0cc726886d2342f6fa6.

So in the case described in that commit, the requested frequency was 455kHz,
but rounding down resulted in a frequency of 476kHz.

That's totally broken and a bad idea.


Sean