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

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Message-ID: <e4a124a8-0905-4504-62e0-a809163775fe@gmx.de>
Date:   Sat, 5 Dec 2020 00:16:01 +0100
From:   Lino Sanfilippo <LinoSanfilippo@....de>
To:     Sean Young <sean@...s.org>
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 Sean,

On 04.12.20 at 09:44, Sean Young wrote:

>> 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;
>

Sound reasonable, I will adjust this.

>
> 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 for this background information.

Regards,
Lino