Messages in this thread |  | | Date | Tue, 31 Mar 2020 13:20:58 -0700 | From | Guru Das Srinagesh <> | Subject | Re: [PATCH v11 06/12] pwm: imx27: Use 64-bit division macro and function |
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On Tue, Mar 31, 2020 at 05:24:52PM +0200, Arnd Bergmann wrote: > On Mon, Mar 30, 2020 at 10:44 PM Guru Das Srinagesh > <gurus@codeaurora.org> wrote: > > > > On Fri, Mar 20, 2020 at 06:09:39PM +0100, Arnd Bergmann wrote: > > > On Fri, Mar 20, 2020 at 2:42 AM Guru Das Srinagesh <gurus@codeaurora.org> wrote: > > > > > > > @@ -240,8 +240,7 @@ static int pwm_imx27_apply(struct pwm_chip *chip, struct pwm_device *pwm, > > > > > > > > period_cycles /= prescale; > > > > c = (unsigned long long)period_cycles * state->duty_cycle; > > > > - do_div(c, state->period); > > > > - duty_cycles = c; > > > > + duty_cycles = div64_u64(c, state->period); > > > > > > > > > > This change looks fine, but I wonder if the code directly above it > > > > > > c = clk_get_rate(imx->clk_per); > > > c *= state->period; > > > do_div(c, 1000000000); > > > period_cycles = c; > > > > > > might run into an overflow when both the clock rate and the period > > > are large numbers. > > > > Hmm. Seems to me like addressing this would be outside the scope of this > > patch series. > > I think it should be part of the same series, addressing bugs that > were introduced > by the change to 64-bit period. If it's not getting fixed along with > the other regressions, > I fear nobody is going to go back and fix it later.
Makes sense, I agree. Would this be an acceptable fix?
Instead of multiplying c and state->period first and then dividing by 10^9, first divide state->period by 10^9 and then multiply the quotient of that division with c and assign it to period_cycles. Like so:
c = clk_get_rate(imx->clk_per); c *= div_u64(state->period, 1000000000); period_cycles = c;
This should take care of overflow not happening because state->period is converted from nanoseconds to seconds early on and so becomes a small number.
Thank you.
Guru Das.
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