Re: [PATCH v7 03/13] hwmon: pwm-fan: Use 64-bit division macros for period and duty cycle

[Uwe Kleine-König]

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

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