linux-kernel - RE: [PATCH v8 01/12] clk: pwm: Use 64-bit division function

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Date:   Thu, 12 Mar 2020 09:14:09 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Guru Das Srinagesh' <gurus@...eaurora.org>
CC:     "linux-pwm@...r.kernel.org" <linux-pwm@...r.kernel.org>,
        Thierry Reding <thierry.reding@...il.com>,
        Uwe Kleine-König <uwe@...ine-koenig.org>,
        Subbaraman Narayanamurthy <subbaram@...eaurora.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Michael Turquette" <mturquette@...libre.com>,
        Stephen Boyd <sboyd@...nel.org>,
        "linux-clk@...r.kernel.org" <linux-clk@...r.kernel.org>
Subject: RE: [PATCH v8 01/12] clk: pwm: Use 64-bit division function

From: Guru Das Srinagesh
> Sent: 12 March 2020 02:10
> On Wed, Mar 11, 2020 at 04:58:24PM +0000, David Laight wrote:
> > From: Guru Das Srinagesh
> > > Sent: 11 March 2020 01:41
> > >
> > > Since the PWM framework is switching struct pwm_args.period's datatype
> > > to u64, prepare for this transition by using div64_u64 to handle a
> > > 64-bit divisor.
> > >
...
> > > --- a/drivers/clk/clk-pwm.c
> > > +++ b/drivers/clk/clk-pwm.c
> > > @@ -89,7 +89,7 @@ static int clk_pwm_probe(struct platform_device *pdev)
> > >  	}
> > >
> > >  	if (of_property_read_u32(node, "clock-frequency", &clk_pwm->fixed_rate))
> > > -		clk_pwm->fixed_rate = NSEC_PER_SEC / pargs.period;
> > > +		clk_pwm->fixed_rate = div64_u64(NSEC_PER_SEC, pargs.period);
> >
> > That cannot be needed, a 32 bit division is fine.
> 
> Could you please explain why? I think the use of this function is
> warranted in order to handle the division properly with a 64-bit
> divisor.
...
> > I'd assign pargs.period to an 'unsigned int' variable
> > prior to the division (I hate casts - been bitten by them in the past.).
> 
> Wouldn't this truncate the 64-bit value? The intention behind this patch
> is to allow the processing of 64-bit values in full.

You are dividing a 32bit constant by a value.
If pargs.period is greater than 2^32 the result is zero.
I think you divide by 'fixed_rate' a bit later on - better not be zero.

	David

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