linux-kernel - Re: About rounding in the clk framework [Was: Re: [PATCH 4/7] pwm: jz4740: Improve algorithm of clock calculation]

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Message-ID: <20201221135758.edgskzkyyrw4lcx7@pengutronix.de>
Date:   Mon, 21 Dec 2020 14:57:58 +0100
From:   Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
To:     Stephen Boyd <sboyd@...nel.org>,
        Michael Turquette <mturquette@...libre.com>
Cc:     linux-pwm@...r.kernel.org, Artur Rojek <contact@...ur-rojek.eu>,
        Mathieu Malaterre <malat@...ian.org>,
        linux-kernel@...r.kernel.org, Paul Cercueil <paul@...pouillou.net>,
        od@...c.me, Thierry Reding <thierry.reding@...il.com>,
        kernel@...gutronix.de, linux-clk@...r.kernel.org
Subject: Re: About rounding in the clk framework [Was: Re: [PATCH 4/7] pwm:
 jz4740: Improve algorithm of clock calculation]

Hello,

On Tue, Apr 14, 2020 at 11:24:12AM +0200, Uwe Kleine-König wrote:
> Hello Stephen, hello Michael,
> 
> On Wed, Feb 12, 2020 at 08:29:11AM +0100, Uwe Kleine-König wrote:
> > Can you please explain what is the reason why clk_round_rate_up/down()
> > is a bad idea? Would it help to create a patch that introduces these
> > functions to get the discussion going?
> 
> I didn't get any feedback on my mail. Are you to busy working on more
> important stuff? Is the answer so obvious that you don't consider it
> worth your time to answer?
> 
> Looking a bit through the code I see there are two callbacks hwclks can
> provide to implement rounding (determine_rate and round_rate). The docs
> for both use the term "return the closes rate actually supported". Does
> that mean "round-closest" is already the official policy and other
> strategies in lowlevel drivers are a bug?

Feedback here would be really appreciated. I intend to unify the rounding
behaviour of PWMs to always round down. If there was a similar
constraint for clks, some corner cases might be a bit simpler.

Looking forward to read about your thoughts,
Uwe

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

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