linux-kernel - Re: [PATCH 00/21] lib: add alternatives for GENMASK()

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Message-ID: <CAHk-=wgVYPp1-06PVU8dZ1_BdXE8MYinviMSX9S+P75kWv3GoA@mail.gmail.com>
Date: Sat, 25 Oct 2025 10:56:24 -0700
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: "Yury Norov (NVIDIA)" <yury.norov@...il.com>
Cc: Linus Walleij <linus.walleij@...aro.org>, 
	Nicolas Frattaroli <nicolas.frattaroli@...labora.com>, 
	Rasmus Villemoes <linux@...musvillemoes.dk>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 00/21] lib: add alternatives for GENMASK()

On Sat, 25 Oct 2025 at 09:40, Yury Norov (NVIDIA) <yury.norov@...il.com> wrote:
>
> GENMASK(high, low) notation reflects a common pattern to describe
> hardware registers. However, out of drivers context it's confusing and
> error-prone.

So I obviously approve of the BITS() syntax, and am not a huge fan of
the odd "GENMASK()" argument ordering.

That said, I'm not convinced about adding the other helpers. I don't
think "FIRST_BITS(8)" is any more readable than "BITS(0,7)", and I
think there's a real danger to having lots of specialized macros that
people then have to know what they mean.

I have an absolutely *disgusting* trick to use the syntax

    BITS(3 ... 25)

to do this all, but it's so disgusting and limited that I don't think
it's actually reasonable.

In case somebody can come up with a cleaner model, here's the trick:

   #define LOWRANGE_0 0,
   #define LOWRANGE_1 1,
   #define LOWRANGE_2 2,
   #define LOWRANGE_3 3,
   #define LOWRANGE_4 4,
   #define LOWRANGE_5 5,
   // ..and so on

   #define _HIGH_VAL(x) (sizeof((char[]){[x]=1})-1)
   #define __FIRST(x, ...) x
   #define ___LOW_VAL(x) __FIRST(x)
   #define __LOW_VAL(x) ___LOW_VAL(LOWRANGE_ ##x)
   #define _LOW_VAL(x) __LOW_VAL(x)

   #define BITS(x) GENMASK(_HIGH_VAL(x), _LOW_VAL(x))

   #define TESTVAL 5
   unsigned long val1 = BITS(3 ... 25);
   unsigned long val2 = BITS(4);
   unsigned long val3 = BITS(TESTVAL ... 31);

and that syntax with either "3 ... 25" or just a plain "4" does
actually work. But only with fairly simple numbers.

It doesn't work with more complex expressions (due to the nasty
preprocessor pasting hack), and I couldn't figure out a way to make it
do so.

I also suspect that we shouldn't do the core code conversions without
having acks from maintainers. Some people may prefer the odd
"high,low" ordering due to it matching some equally odd hardware
documentation.

               Linus