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

[Guenter Roeck]

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