linux-kernel - Re: [PATCH treewide v3 2/4] bitfield: Add non-constant field

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Message-ID: <20251017103936.49fbafdd@kernel.org>
Date: Fri, 17 Oct 2025 10:39:36 -0700
From: Jakub Kicinski <kuba@...nel.org>
To: Geert Uytterhoeven <geert@...ux-m68k.org>
Cc: Michael Turquette <mturquette@...libre.com>, Stephen Boyd
 <sboyd@...nel.org>, Nicolas Ferre <nicolas.ferre@...rochip.com>, Alexandre
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 linux-kernel@...r.kernel.org, Jonathan Cameron
 <Jonathan.Cameron@...wei.com>
Subject: Re: [PATCH treewide v3 2/4] bitfield: Add non-constant
 field_{prep,get}() helpers

On Fri, 17 Oct 2025 18:00:41 +0200 Geert Uytterhoeven wrote:
> > +/**
> > + * u32_encode_bits() - prepare a u32 bitfield element (non-const)
> > + * @v: value to put in the field
> > + * @field: shifted mask defining the field's length and position
> > + *
> > + * Equivalent of FIELD_PREP() for u32, field does not have to be constant.
> > + *
> > + * Note that the helper is available for other field widths (generated below).
> > + */
> > +static __always_inline __u32 u32_encode_bits(u32 v, u32 field)
> > +{
> > +       if (__builtin_constant_p(v) && (v & ~field_mask(field)))
> > +               __field_overflow();
> > +       return ((v & field_mask(field)) * field_multiplier(field));  
> 
> Unfortunately gcc emits actual divisions or __*div*() calls, and
> multiplications in the non-constant case.
> 
> So I don't think this is suitable as-is.

Sorry I missed or forgot that you replied :(

The inline helpers exist already have have a lot of uses. If __ffs is
more optimal then why not make existing helpers use it as well? 
It'd be far more beneficial:

$ git grep u32_encode_bits | wc -l
391

Sorry if I'm being slow..