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

[Rasmus Villemoes <ravi@prevas.dk>]

On Sat, Oct 25 2025, Linus Torvalds <torvalds@linux-foundation.org> wrote:

> On Sat, 25 Oct 2025 at 09:40, Yury Norov (NVIDIA) <yury.norov@gmail.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.

It's cute, but no, I also don't know how to make it work without the
preprocessor concatenation stuff.

There is, however, an alternative that resembles the syntax in data
sheets even more closely:

#define BITS(low_high) GENMASK((0 ? low_high), (1 ? low_high))

That'll work for

unsigned long val1 = BITS(3:25);
unsigned long val3 = BITS(TESTVAL:31);

for most of the things TESTVAL might expand to - I'm not sure what would
happen if one of the lo/hi values contains ternaries and isn't properly
parenthesized, but nobody writes stuff like

#define RESET_BIT(rev) rev == 0xaa ? 7 : 9

It doesn't work for the single-bit case, but I don't think it's so bad
to have to say

unsigned long val2 = BIT(4);

and obviously BITS(4:4) would work as well.

It also doesn't do anything to prevent the hi-lo 11:7 order.

Rasmus