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

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Message-ID: <20201206141941.GA24807@gofer.mess.org>
Date:   Sun, 6 Dec 2020 14:19:41 +0000
From:   Sean Young <sean@...s.org>
To:     Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>
Cc:     Lino Sanfilippo <LinoSanfilippo@....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

Hello Uwe,

On Sat, Dec 05, 2020 at 08:25:10PM +0100, Uwe Kleine-König wrote:
> On Sat, Dec 05, 2020 at 05:34:44PM +0000, Sean Young wrote:
> > What real life uses-cases are there for round down? If you want to round
> > down, is there any need for round up?
> 
> The scenario I have in mind is for driving a motor. I have to admit
> however that usually the period doesn't matter much and it's the
> duty_cycle that defines the motor's speed. So for this case the
> conservative behaviour is round-down to not make the motor run faster
> than expected.

I am reading here that for driving motors, only the duty cycle matters,
not the period.

> For other usecases (fan, backlight, LED) exactness typically doesn't
> matter that much.

So, the use-cases you have are driving motor, fan, backlight, and led.
And in all these cases the exact Hz does not matter.

The only uses case where the exact Hz does matter is pwm-ir-tx. 

So, I gather there are no use-cases for round-down. Yes, should round-down
be needed, then this is more difficult to implement if the driver always
does a round-closest. But, since there is no reason to have round-down,
this is all academic.

Your policy of forcing new pwm drivers to use round-down is breaking
pwm-ir-tx.

Thanks,

Sean