linux-kernel - Re: [PATCH v2 1/3] units: Add SI metric prefix definitions

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Message-ID: <ae919bae6c21e23b0a1ed0c9327738e59dd90aa0.camel@perches.com>
Date:   Mon, 07 Jun 2021 12:09:58 -0700
From:   Joe Perches <joe@...ches.com>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        linux-i2c@...r.kernel.org, linux-kernel@...r.kernel.org
Cc:     Jarkko Nikula <jarkko.nikula@...ux.intel.com>,
        Mika Westerberg <mika.westerberg@...ux.intel.com>,
        wsa@...nel.org
Subject: Re: [PATCH v2 1/3] units: Add SI metric prefix definitions

On Mon, 2021-06-07 at 18:23 +0300, Andy Shevchenko wrote:
> Sometimes it's useful to have well-defined SI metric prefix to be used
> to self-describe the formulas or equations.
> 
> List most popular ones in the units.h.

trivia:

> diff --git a/include/linux/units.h b/include/linux/units.h
[]
> @@ -4,6 +4,22 @@
>  
> 
>  #include <linux/math.h>
>  
> +/* Metric prefixes in accordance with Système international (d'unités) */
> +#define PETA	1000000000000000LL
> +#define TERA	1000000000000LL
> +#define GIGA	1000000000L
> +#define MEGA	1000000L
> +#define KILO	1000L

It's perhaps more legible to avoid the long sequences of zeros by
using preprocessor mechanisms.

#define KILO (1000UL)
#define MEGA (KILO * 1000)
#define GIGA (MEGA * 1000)
#define TERA (GIGA * 1000ULL)
#define PETA (TERA * 1000)

or maybe:

#define KILO (1000UL)
#define MEGA (1000UL * 1000UL)
#define GIGA (1000UL * 1000UL * 1000UL)
#define TERA (1000ULL * 1000ULL * 1000ULL * 1000ULL)
#define PETA (1000ULL * 1000ULL * 1000ULL * 1000ULL * 1000ULL)

etc...

> +#define HECTO 100L
> +#define DECA 10L

Should it be DECA or DEKA or both?

https://www.nist.gov/pml/weights-and-measures/metric-si-prefixes
deka Example: dekameter da 	10**1 	Ten

> +#define DECI	10L
> +#define CENTI	100L
> +#define MILLI	1000L
> +#define MICRO	1000000L
> +#define NANO	1000000000L
> +#define PICO	1000000000000LL
> +#define FEMTO	1000000000000000LL

IMO: Might as well include all the prefixes up to 10**24

EXA ZETTA YOTTA, ATTA ZEPTO YOCTO

And how do people avoid using MILLI for KILO, MEGA for MICRO, etc...
The compiler won't care but usage could look odd.