linux-kernel - Re: [PATCH v2] pwm: imx-tpm: Use correct MODULO value for EPWM mode

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Message-ID: <6d884afde9d41b6efff7be5f820f8121f38e8321.camel@iris-sensing.com>
Date: Fri, 25 Oct 2024 07:25:50 +0000
From: Erik Schumacher <erik.schumacher@...s-sensing.com>
To: "u.kleine-koenig@...libre.com" <u.kleine-koenig@...libre.com>
CC: "kernel@...gutronix.de" <kernel@...gutronix.de>,
	"thierry.reding@...il.com" <thierry.reding@...il.com>,
	"s.hauer@...gutronix.de" <s.hauer@...gutronix.de>, "festevam@...il.com"
	<festevam@...il.com>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "shawnguo@...nel.org" <shawnguo@...nel.org>,
	"linux-arm-kernel@...ts.infradead.org"
	<linux-arm-kernel@...ts.infradead.org>, "imx@...ts.linux.dev"
	<imx@...ts.linux.dev>, "linux-pwm@...r.kernel.org"
	<linux-pwm@...r.kernel.org>
Subject: Re: [PATCH v2] pwm: imx-tpm: Use correct MODULO value for EPWM mode

Hello,

thanks for the review.

Am Donnerstag, dem 24.10.2024 um 23:01 +0200 schrieb Uwe Kleine-König:
> Hello,
> 
> [dropping anson.huang@....com from Cc]
> 
> On Thu, Oct 24, 2024 at 06:41:07AM +0000, Erik Schumacher wrote:
> > The modulo register defines the period of the edge-aligned PWM mode
> > (which is the only mode implemented). The reference manual states:
> > "The EPWM period is determined by (MOD + 0001h) ..." So the value that
> > is written to the MOD register must therefore be one less than the
> > calculated period length.
> > A correct MODULO value is particularly relevant if the PWM has to output
> > a high frequency due to a low period value.
> > 
> > Fixes: 738a1cfec2ed ("pwm: Add i.MX TPM PWM driver support")
> > Cc: stable@...r.kernel.org
> > 
> > Signed-off-by: Erik Schumacher <erik.schumacher@...s-sensing.com>
> 
> No empty line between these trailer lines please.

Noted for v3!

> 
> > diff --git a/drivers/pwm/pwm-imx-tpm.c b/drivers/pwm/pwm-imx-tpm.c
> > index 96ea343856f0..a05b66ffe208 100644
> > --- a/drivers/pwm/pwm-imx-tpm.c
> > +++ b/drivers/pwm/pwm-imx-tpm.c
> > @@ -106,7 +106,7 @@ static int pwm_imx_tpm_round_state(struct pwm_chip *chip,
> >  	p->prescale = prescale;
> >  
> >  	period_count = (clock_unit + ((1 << prescale) >> 1)) >> prescale;
> > -	p->mod = period_count;
> > +	p->mod = period_count - 1;
> 
> This looks bogus if period_count is 0.
> 

That's true. It can only be 0 if someone is requesting a period from
the PWM that is lower than the period of the clock feeding the PWM.
But, it would be an invalid config and returning -EINVAL there seems to
be the right thing (instead of underflowing the integer). I will add
such a check.

> Best regards
> Uwe

Kind regards
Erik