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Date:   Tue, 10 Mar 2020 13:08:14 +0100
From:   Uwe =?iso-8859-1?Q?Kleine-K=F6nig?= 
        <u.kleine-koenig@pengutronix.de>
To:     Guenter Roeck <linux@roeck-us.net>
Cc:     Guru Das Srinagesh <gurus@codeaurora.org>,
        linux-pwm@vger.kernel.org,
        Thierry Reding <thierry.reding@gmail.com>,
        Subbaraman Narayanamurthy <subbaram@codeaurora.org>,
        linux-kernel@vger.kernel.org, Kamil Debski <kamil@wypas.org>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>,
        Jean Delvare <jdelvare@suse.com>,
        Liam Girdwood <lgirdwood@gmail.com>,
        Mark Brown <broonie@kernel.org>, linux-hwmon@vger.kernel.org
Subject: Re: [PATCH v7 03/13] hwmon: pwm-fan: Use 64-bit division macros for
 period and duty cycle
Message-ID: <20200310120814.4kjxmii3c7zxw55y@pengutronix.de>
References: <cover.1583782035.git.gurus@codeaurora.org>
 <b503833e0f58bd6dd9fe84d866124e7c457e099e.1583782035.git.gurus@codeaurora.org>
 <20200309214822.GA19773@roeck-us.net>
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Hello Guenter,

On Mon, Mar 09, 2020 at 02:48:22PM -0700, Guenter Roeck wrote:
> On Mon, Mar 09, 2020 at 12:35:06PM -0700, Guru Das Srinagesh wrote:
> > Because period and duty cycle are defined in the PWM framework structs
> > as ints with units of nanoseconds, the maximum time duration that can be
> > set is limited to ~2.147 seconds. Redefining them as u64 values will
> > enable larger time durations to be set.
> > 
> > As a first step, prepare drivers to handle the switch to u64 period and
> > duty_cycle by replacing division operations involving pwm period and duty cycle
> > with their 64-bit equivalents as appropriate. The actual switch to u64 period
> > and duty_cycle follows as a separate patch.
> > 
> > Where the dividend is 64-bit but the divisor is 32-bit, use *_ULL
> > macros:
> > - DIV_ROUND_UP_ULL
> > - DIV_ROUND_CLOSEST_ULL
> > - div_u64
> > 
> > Where the divisor is 64-bit (dividend may be 32-bit or 64-bit), use
> > DIV64_* macros:
> > - DIV64_U64_ROUND_CLOSEST
> > - div64_u64
>
> There is no explanation why this is necessary. What is the use case ?
> It is hard to imagine a real-world use case with a duty cycle of more
> than 2 seconds.

When my Laptop is in suspend there is an LED that blinks with a period
of approximately 5 seconds. (To be fair, the brightness is more a sinus
than a rectangle, but still.)

Best regards
Uwe

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