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Date:   Mon, 9 Mar 2020 14:48:22 -0700
From:   Guenter Roeck <linux@roeck-us.net>
To:     Guru Das Srinagesh <gurus@codeaurora.org>
Cc:     linux-pwm@vger.kernel.org,
        Thierry Reding <thierry.reding@gmail.com>,
        Uwe =?iso-8859-1?Q?Kleine-K=F6nig?= <uwe@kleine-koenig.org>,
        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: <20200309214822.GA19773@roeck-us.net>
References: <cover.1583782035.git.gurus@codeaurora.org>
 <b503833e0f58bd6dd9fe84d866124e7c457e099e.1583782035.git.gurus@codeaurora.org>
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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.

Guenter