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Date: Thu, 19 Mar 2020 13:51:17 -0700 From: Guru Das Srinagesh <gurus@...eaurora.org> To: Arnd Bergmann <arnd@...db.de> Cc: Linux PWM List <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>, Alexander Shiyan <shc_work@...l.ru> Subject: Re: [PATCH v9 04/11] pwm: clps711x: Use 64-bit division macro On Wed, Mar 18, 2020 at 08:38:29PM +0100, Arnd Bergmann wrote: > On Wed, Mar 18, 2020 at 6:00 PM Guru Das Srinagesh <gurus@...eaurora.org> wrote: > > > > On Wed, Mar 18, 2020 at 10:49:34AM +0100, Arnd Bergmann wrote: > > > On Wed, Mar 18, 2020 at 12:30 AM Guru Das Srinagesh > > > <gurus@...eaurora.org> wrote: > > > > > > > > On Tue, Mar 17, 2020 at 11:22:06PM +0100, Arnd Bergmann wrote: > > > > > > diff --git a/drivers/pwm/pwm-clps711x.c b/drivers/pwm/pwm-clps711x.c > > > > > > index 924d39a..ba9500a 100644 > > > > > > --- a/drivers/pwm/pwm-clps711x.c > > > > > > +++ b/drivers/pwm/pwm-clps711x.c > > > > > > @@ -43,7 +43,7 @@ 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); > > > > > > + return DIV64_U64_ROUND_CLOSEST(v * 0xf, pwm->args.period); > > > > > > } > > > > > > > > > > Is it actually going to exceed U32_MAX? If not, a type cast may be > > > > > more appropriate here than the expensive 64-bit division. > > > > > > > > With the final change in this patch series, the framework will support > > > > periods that exceed U32_MAX. My concern is that using a typecast would > > > > mean that in those cases, this driver will not support > U32_MAX values. > > > > Using DIV64_U64_ROUND_CLOSEST makes the driver future proof and able to > > > > handle > U32_MAX values correctly. What do you think? > > > > > > Ah, so if the period can actually be larger than U32_MAX, you need to > > > handle that case. However, I see that the divident in this code (v * 0xf) > > > is still a 32-bit number, so a correct and efficient implementation could be > > > > > > if (pwm->args.period > (UINT_MAX / 0xf)) > > > > Shouldn't the if condition be the following? Or am I missing > > something here? > > > > if (pwm->args.period > (UINT_MAX / (v * 0xf))) > > ^^^^^^^^^ > > That would require performing a division, which is an external function > call on ARMv4. My version just checks for an upper bound and completely > avoids the division. You could also just check for ">UINT_MAX" if you > find that clearer. Thanks, have checked for UINT_MAX in v10 of my patchset that I just uploaded. Could you please review it? Thank you. Guru Das.
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