Re: [PATCH v18 3/4] pwm: Add support for RZ/G2L GPT - Uwe Kleine-König

From: "Uwe Kleine-König" <u.kleine-koenig@pengutronix.de>
To: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Biju Das <biju.das.au@gmail.com>,
	linux-pwm@vger.kernel.org,  kernel@pengutronix.de,
	Magnus Damm <magnus.damm@gmail.com>,
	 Prabhakar Mahadev Lad <prabhakar.mahadev-lad.rj@bp.renesas.com>,
	Fabrizio Castro <fabrizio.castro.jz@renesas.com>,
	 linux-renesas-soc@vger.kernel.org,
	Thierry Reding <thierry.reding@gmail.com>,
	 Philipp Zabel <p.zabel@pengutronix.de>,
	Biju Das <biju.das.jz@bp.renesas.com>
Subject: Re: [PATCH v18 3/4] pwm: Add support for RZ/G2L GPT
Date: Tue, 12 Mar 2024 10:27:51 +0100	[thread overview]
Message-ID: <fdbdfaaczwiqweoa6lot7rycjnhc3fsvsluwx3c5mwc6ic2dsv@m6foz6somnuz> (raw)
In-Reply-To: <CAMuHMdV8SnMgawrMspemJNfsAHW-wSboXeEOgZ6F37QqrmiLSA@mail.gmail.com>

[-- Attachment #1: Type: text/plain, Size: 2135 bytes --]

Hello Geert,

On Tue, Mar 12, 2024 at 09:11:42AM +0100, Geert Uytterhoeven wrote:
> On Tue, Mar 12, 2024 at 8:20 AM Uwe Kleine-König
> <u.kleine-koenig@pengutronix.de> wrote:
> > On Tue, Feb 20, 2024 at 07:43:17PM +0000, Biju Das wrote:
> > > --- /dev/null
> > > +++ b/drivers/pwm/pwm-rzg2l-gpt.c
> > > +static u64 calculate_period_or_duty(struct rzg2l_gpt_chip *rzg2l_gpt, u32 val, u8 prescale)
> > > +{
> > > +     u64 tmp, d;
> > > +
> > > +     /*
> > > +      * Rate is in MHz and is always integer for peripheral clk
> > > +      * 2^32 * 2^10 (prescalar) * 10^9 > 2^64
> > > +      * 2^32 * 2^10 (prescalar) * 10^6 < 2^64
> > > +      * Multiply val with prescalar first, if the result is less than
> > > +      * 2^34, then multiply by 10^9. Otherwise divide nr and dr by 10^3
> > > +      * so that it will never overflow.
> > > +      */
> > > +
> > > +     tmp = (u64)val << (2 * prescale);
> > > +     if (tmp <= (1ULL << 34)) {
> >
> > I would have written that as:
> >
> >         if (tmp >> 34 == 0)
> >
> > (which implements tmp < (1ULL << 34), which doesn't matter much).
> >
> > > +             tmp *= NSEC_PER_SEC;
> > > +             d = rzg2l_gpt->rate;
> > > +     } else {
> > > +             tmp *= div64_u64(NSEC_PER_SEC, KILO);
> >
> > I don't know if the compiler is clever enough to not calculate that
> > every time?
> 
> Not on 32-bit when written that way.
> 
> > Also using div64_u64 is too heavy given that both values fit
> > into an u32.
> 
> Indeed, so "NSEC_PER_SEC / KILO" should be fine.

ack.

> I guess NSEC_PER_MSEC would be too obfuscating?

or USEC_PER_SEC? Not sure. Also I'm unsure if using KILO instead of 1000
is really an improvement.

If you know that the clkrate is a multiple of 1000 there is no reason to
not use clkrate / 1000 unconditionally. So maybe use
rzg2l_gpt->rate_kHz (and error out in .probe if the rate isn't a
multiple of 1000?)

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | https://www.pengutronix.de/ |

[-- Attachment #2: signature.asc --]
[-- Type: application/pgp-signature, Size: 488 bytes --]