linux-kernel - Re: [PATCH v11 11/12] clk: pwm: Assign u64 divisor to unsigned int before use

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Message-ID: <20200407024013.GB7019@codeaurora.org>
Date:   Mon, 6 Apr 2020 19:40:13 -0700
From:   Guru Das Srinagesh <gurus@...eaurora.org>
To:     David Laight <David.Laight@...LAB.COM>
Cc:     'Arnd Bergmann' <arnd@...db.de>,
        Linux PWM List <linux-pwm@...r.kernel.org>,
        Thierry Reding <thierry.reding@...il.com>,
        Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>,
        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 <linux-clk@...r.kernel.org>
Subject: Re: [PATCH v11 11/12] clk: pwm: Assign u64 divisor to unsigned int
 before use

On Fri, Mar 20, 2020 at 06:42:39PM +0000, David Laight wrote:
> From: Arnd Bergmann
> > Sent: 20 March 2020 17:01
> > On Fri, Mar 20, 2020 at 2:42 AM Guru Das Srinagesh <gurus@...eaurora.org> wrote:
> > >
> > > Since the PWM framework is switching struct pwm_args.period's datatype
> > > to u64, prepare for this transition by assigning the 64-bit divisor to
> > > an unsigned int variable to use as the divisor. This is being done
> > > because the divisor is a 32-bit constant and the quotient will be zero
> > > if the divisor exceeds 2^32.

Correction: The quotient will be zero when the denominator exceeds the
numerator, i.e. NSECS_PER_SEC, and not U32_MAX. For this to happen, the
property "clock-frequency" must be specified to be more than
NSEC_PER_SEC, i.e. 1 GHz. Just observed that currently in the device
tree, all instances of this driver (compatible string "pwm-clock") are
setting this property to values within that limit.

> > >
> > > Cc: Michael Turquette <mturquette@...libre.com>
> > > Cc: Stephen Boyd <sboyd@...nel.org>
> > > Cc: linux-clk@...r.kernel.org
> > > Cc: David Laight <David.Laight@...LAB.COM>
> > >
> > > Reported-by: kbuild test robot <lkp@...el.com>
> > > Signed-off-by: Guru Das Srinagesh <gurus@...eaurora.org>
> > > ---
> > >  drivers/clk/clk-pwm.c | 4 +++-
> > >  1 file changed, 3 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/drivers/clk/clk-pwm.c b/drivers/clk/clk-pwm.c
> > > index 87fe0b0e..c0b5da3 100644
> > > --- a/drivers/clk/clk-pwm.c
> > > +++ b/drivers/clk/clk-pwm.c
> > > @@ -72,6 +72,7 @@ static int clk_pwm_probe(struct platform_device *pdev)
> > >         struct pwm_device *pwm;
> > >         struct pwm_args pargs;
> > >         const char *clk_name;
> > > +       unsigned int period;
> > >         int ret;
> > >
> > >         clk_pwm = devm_kzalloc(&pdev->dev, sizeof(*clk_pwm), GFP_KERNEL);
> > > @@ -88,8 +89,9 @@ static int clk_pwm_probe(struct platform_device *pdev)
> > >                 return -EINVAL;
> > >         }
> > >
> > > +       period = pargs.period;
> > >         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 = NSEC_PER_SEC / period;
> > >
> > >         if (pargs.period != NSEC_PER_SEC / clk_pwm->fixed_rate &&
> > >             pargs.period != DIV_ROUND_UP(NSEC_PER_SEC, clk_pwm->fixed_rate)) {
> > 
> > Doesn't this one need a check for "pargs.period>UINT_MAX" or
> > "pargs.period > NSEC_PER_SEC"?
> > 

With the assignment of period to unsigned int, wouldn't doing
s/pargs.period/period suffice?

Also, will add a check to ensure that clk_pwm->fixed_rate is non-zero. If it
is zero, fail probe.

> > It looks like truncating the 64-bit value to a 32-bit type can result in
> > unexpected behavior.
> 
> I also suspect the last two lines ought to use the 32bit copy.
> And there is a chance that the division will explode.

The check mentioned above will ensure that the division will not
explode.

What do you guys think?

Thank you.

Guru Das.