linux-kernel - Re: [RESEND PATCH v14 04/11] pwm: clps711x: Cast period to u32 before use as divisor

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20200522093738.cko5rj4wrxfd4hxu@holly.lan>
Date:   Fri, 22 May 2020 10:37:38 +0100
From:   Daniel Thompson <daniel.thompson@...aro.org>
To:     Guru Das Srinagesh <gurus@...eaurora.org>
Cc:     linux-pwm@...r.kernel.org,
        Thierry Reding <thierry.reding@...il.com>,
        Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>,
        Subbaraman Narayanamurthy <subbaram@...eaurora.org>,
        David Collins <collinsd@...eaurora.org>,
        linux-kernel@...r.kernel.org, Joe Perches <joe@...ches.com>,
        Stephen Boyd <sboyd@...nel.org>,
        Lee Jones <lee.jones@...aro.org>,
        Arnd Bergmann <arnd@...db.de>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Guenter Roeck <linux@...ck-us.net>,
        Dan Carpenter <dan.carpenter@...cle.com>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [RESEND PATCH v14 04/11] pwm: clps711x: Cast period to u32
 before use as divisor

On Thu, May 21, 2020 at 01:25:25PM -0700, Guru Das Srinagesh wrote:
> On Thu, May 21, 2020 at 11:19:34AM +0100, Daniel Thompson wrote:
> > On Wed, May 20, 2020 at 03:55:57PM -0700, Guru Das Srinagesh wrote:
> > > Since the PWM framework is switching struct pwm_args.period's datatype
> > > to u64, prepare for this transition by typecasting it to u32.
> > > 
> > > Also, since the dividend is still a 32-bit number, any divisor greater
> > > than the numerator will cause the quotient to be zero, so return 0 in
> > > that case to efficiently skip the division.
> > > 
> > > Signed-off-by: Guru Das Srinagesh <gurus@...eaurora.org>
> > > ---
> > >  drivers/pwm/pwm-clps711x.c | 5 ++++-
> > >  1 file changed, 4 insertions(+), 1 deletion(-)> > > 
> > > diff --git a/drivers/pwm/pwm-clps711x.c b/drivers/pwm/pwm-clps711x.c
> > > index 924d39a..da771b1 100644
> > > --- a/drivers/pwm/pwm-clps711x.c
> > > +++ b/drivers/pwm/pwm-clps711x.c
> > > @@ -43,7 +43,10 @@ static void clps711x_pwm_update_val(struct clps711x_chip *priv, u32 n, u32 v)
> > >  static unsigned int clps711x_get_duty(struct pwm_device *pwm, unsigned int v)
> > >  {
> > >  	/* Duty cycle 0..15 max */
> > > -	return DIV_ROUND_CLOSEST(v * 0xf, pwm->args.period);
> > > +	if (pwm->args.period > (v * 0xf))
> > > +		return 0;
> > 
> > This doesn't look right to me.
> > 
> > DIV_ROUND_CLOSEST() does rounded division and the short circuit doesn't
> > implement that.
> 
> My initial patch [1] was to simply use DIV64_U64_ROUND_CLOSEST(), but I
> got review feedback to add a short-circuit (same thread, [2]). I feel
> like I should skip the short-circuiting and type casting and simply just
> use DIV64_U64_ROUND_CLOSEST() - what do you think?

A trivial review of pwm-clps711x.c suggests that the period is always
32-bit anyway so why not just throw away the short circuit entirely and
replace with a comment saying that CLPS711X has a hard coded period
that is always >1000000000 ?


Daniel.