Re: [PATCH 6/6] pwm: renesas-tpu: Improve precision of period and duty_cycle calculation

From: Geert Uytterhoeven <geert@linux-m68k.org>
To: "Uwe Kleine-König" <u.kleine-koenig@pengutronix.de>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>,
	Simon Horman <horms+renesas@verge.net.au>,
	Magnus Damm <magnus.damm@gmail.com>,
	Thierry Reding <thierry.reding@gmail.com>,
	Lee Jones <lee.jones@linaro.org>,
	Linux PWM List <linux-pwm@vger.kernel.org>,
	Linux-Renesas <linux-renesas-soc@vger.kernel.org>,
	Sascha Hauer <kernel@pengutronix.de>
Subject: Re: [PATCH 6/6] pwm: renesas-tpu: Improve precision of period and duty_cycle calculation
Date: Tue, 19 Apr 2022 09:41:45 +0200	[thread overview]
Message-ID: <CAMuHMdV7okyXpM=q1AMiv1xZSWPGpL1Kz0wzQSAtcKdfE88+kQ@mail.gmail.com> (raw)
In-Reply-To: <CAMuHMdX7Kic2WG+f6hAdjz7SGSjWfi-UTdOrxtmV6D4zE3zh1Q@mail.gmail.com>

Hi Uwe,

On Thu, Apr 14, 2022 at 12:27 PM Geert Uytterhoeven
<geert@linux-m68k.org> wrote:
> On Wed, Apr 13, 2022 at 10:51 AM Uwe Kleine-König
> <u.kleine-koenig@pengutronix.de> wrote:
> > Dividing by the result of a division looses precision. Consider for example
> > clk_rate = 33000000 and period_ns = 500001. Then
> >
> >         clk_rate / (NSEC_PER_SEC / period_ns)
> >
> > has the exact value 16500.033, but in C this evaluates to 16508. It gets
> > worse for even bigger values of period_ns, so with period_ns = 500000001,
> > the exact result is 16500000.033 while in C we get 33000000.
> >
> > For that reason use
> >
> >         clk_rate * period_ns / NSEC_PER_SEC
> >
> > instead which doesn't suffer from this problem. To ensure this doesn't
> > overflow add a safeguard check for clk_rate.
> >
> > Incidentally this fixes a division by zero if period_ns > NSEC_PER_SEC.
> > Another side effect is that values bigger than INT_MAX for period and
> > duty_cyle are not wrongly discarded any more.
>
> You forgot to mention that pwm_state.period is no longer truncated to u32.

Please ignore this bogus comment.
Sorry for the fuzz.

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds