linux-kernel - Re: [PATCH v2 2/6] units: Add 32- and 64-bit signed values of π

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Message-ID: <20251109170032.2763ae5b@jic23-huawei>
Date: Sun, 9 Nov 2025 17:00:32 +0000
From: Jonathan Cameron <jic23@...nel.org>
To: Andy Shevchenko <andy.shevchenko@...il.com>
Cc: David Lechner <dlechner@...libre.com>, Andy Shevchenko
 <andriy.shevchenko@...ux.intel.com>, Hans Verkuil
 <hverkuil+cisco@...nel.org>, "linux-iio@...r.kernel.org"
 <linux-iio@...r.kernel.org>, "chrome-platform@...ts.linux.dev"
 <chrome-platform@...ts.linux.dev>, "linux-kernel@...r.kernel.org"
 <linux-kernel@...r.kernel.org>, "linux-media@...r.kernel.org"
 <linux-media@...r.kernel.org>, Nuno Sá <nuno.sa@...log.com>,
 Andy Shevchenko <andy@...nel.org>, Benson Leung <bleung@...omium.org>,
 Guenter Roeck <groeck@...omium.org>, Lars-Peter Clausen <lars@...afoo.de>,
 Michael Hennerich <Michael.Hennerich@...log.com>, Mauro Carvalho Chehab
 <mchehab@...nel.org>, "Daniel W. S. Almeida" <dwlsalmeida@...il.com>
Subject: Re: [PATCH v2 2/6] units: Add 32- and 64-bit signed values of
 π

On Sat, 8 Nov 2025 21:02:54 +0200
Andy Shevchenko <andy.shevchenko@...il.com> wrote:

> пʼятниця, 7 листопада 2025 р. David Lechner <dlechner@...libre.com> пише:
> 
> > On 11/7/25 2:03 PM, Andy Shevchenko wrote:  
> > > There are a few drivers that want to have these values, and
> > > one more known to come soon. Let's define the values for them.
> > >  
> 
> 
> 
> 
> 
> > > @@ -21,6 +21,12 @@
> > >  #define PICO 1000000000000ULL
> > >  #define FEMTO        1000000000000000ULL
> > >
> > > +/* Value of π * 10⁸ (fits s32 or signed int) */
> > > +#define PI   314159265  
> >
> > It isn't clear to me at all in the other patches that e.g.
> > DIV_ROUND_UP(PI, 1000) would be π ✕ 10⁴ (rounded to nearest
> > integer, of course).
> >
> > Calling these PI_E8 and PI_E18 or PI_x10_8 and PI_x10_18
> > would help to clear that up.  
> 
> 
> 
> This will be an awful and ugly name(s). I fully disagree on such a
> proposal. The power is chosen to fit the type with maximum precision, no
> need to explain this in the name.
With no indicate of the multiplier we have to check it every time wwe
use it which isn't ideal either.

So I agree with David that we have to indicate this somewhere. Maybe hold the
multiplier in a separate define
#define PI_MUL
#define PI_MUL_LL

Then compute the divisions by (PI_MUL) / KILO or something along those lines?
> 
> >  
> > > +
> > > +/* Value of π * 10¹⁸ (fits s64 or signed long long) */
> > > +#define PI_LL        3141592653589793238LL
> > > +
> > >  /* Hz based multipliers */
> > >  #define NANOHZ_PER_HZ                1000000000UL
> > >  #define MICROHZ_PER_HZ               1000000UL  
> >
> >