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

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Date:   Wed, 8 Apr 2020 19:40:45 -0700
From:   Guru Das Srinagesh <gurus@...eaurora.org>
To:     David Laight <David.Laight@...LAB.COM>
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

On Thu, Mar 12, 2020 at 09:14:09AM +0000, David Laight wrote:
> 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.

Correction: if pargs.period is greater than NSEC_PER_SEC, not 2^32.

> I think you divide by 'fixed_rate' a bit later on - better not be zero.

I am adding an explicit check in v12 to ensure fixed_rate is not zero. If
during the calculation it is found to be zero, probing will fail.

I think with this modification, this v8 version of this change makes
sense to use.

Thank you.

Guru Das.