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

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Date:   Wed, 11 Mar 2020 19:09:40 -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 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.
> > 
> > Cc: Michael Turquette <mturquette@...libre.com>
> > Cc: Stephen Boyd <sboyd@...nel.org>
> > Cc: linux-clk@...r.kernel.org
> > 
> > Signed-off-by: Guru Das Srinagesh <gurus@...eaurora.org>
> > ---
> >  drivers/clk/clk-pwm.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/drivers/clk/clk-pwm.c b/drivers/clk/clk-pwm.c
> > index 87fe0b0e..7b1f7a0 100644
> > --- 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.

> More interesting would be whether pargs.period is sane (eg not zero).

There is a non-zero check for pargs.period just prior to this line, so
the code is handling this case already.

> 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.

Thank you.

Guru Das.