linux-kernel - Re: [PATCH 3/9] bitmap: Use FIELD_PREP() in expansion of FIELD_PREP

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Message-ID: <6719438.lOV4Wx5bFT@workhorse>
Date: Thu, 11 Dec 2025 13:50:28 +0100
From: Nicolas Frattaroli <nicolas.frattaroli@...labora.com>
To: David Laight <david.laight.linux@...il.com>
Cc: Yury Norov <yury.norov@...il.com>,
 Rasmus Villemoes <linux@...musvillemoes.dk>, linux-kernel@...r.kernel.org,
 linux-usb@...r.kernel.org, Geert Uytterhoeven <geert+renesas@...der.be>,
 Alexandre Belloni <alexandre.belloni@...tlin.com>,
 Jonathan Cameron <Jonathan.Cameron@...wei.com>, Crt Mori <cmo@...exis.com>,
 Richard Genoud <richard.genoud@...tlin.com>,
 Andy Shevchenko <andriy.shevchenko@...el.com>,
 Luo Jie <quic_luoj@...cinc.com>, Peter Zijlstra <peterz@...radead.org>,
 Jakub Kicinski <kuba@...nel.org>, netdev@...r.kernel.org,
 "David S . Miller" <davem@...emloft.net>,
 Simon Horman <simon.horman@...ronome.com>,
 Mika Westerberg <mika.westerberg@...ux.intel.com>,
 Andreas Noever <andreas.noever@...il.com>,
 Yehezkel Bernat <YehezkelShB@...il.com>
Subject:
 Re: [PATCH 3/9] bitmap: Use FIELD_PREP() in expansion of FIELD_PREP_WM16()

On Wednesday, 10 December 2025 21:59:15 Central European Standard Time David Laight wrote:
> On Wed, 10 Dec 2025 20:18:30 +0100
> Nicolas Frattaroli <nicolas.frattaroli@...labora.com> wrote:
> 
> > On Tuesday, 9 December 2025 11:03:07 Central European Standard Time david.laight.linux@...il.com wrote:
> > > From: David Laight <david.laight.linux@...il.com>
> > > 
> > > Instead of directly expanding __BF_FIELD_CHECK() (which really ought
> > > not be used outside bitfield) and open-coding the generation of the
> > > masked value, just call FIELD_PREP() and add an extra check for
> > > the mask being at most 16 bits.
> > > 
> > > Signed-off-by: David Laight <david.laight.linux@...il.com>
> > > ---
> > >  include/linux/hw_bitfield.h | 17 ++++++++---------
> > >  1 file changed, 8 insertions(+), 9 deletions(-)
> > > 
> > > diff --git a/include/linux/hw_bitfield.h b/include/linux/hw_bitfield.h
> > > index df202e167ce4..d7f21b60449b 100644
> > > --- a/include/linux/hw_bitfield.h
> > > +++ b/include/linux/hw_bitfield.h
> > > @@ -23,15 +23,14 @@
> > >   * register, a bit in the lower half is only updated if the corresponding bit
> > >   * in the upper half is high.
> > >   */
> > > -#define FIELD_PREP_WM16(_mask, _val)					     \
> > > -	({								     \
> > > -		typeof(_val) __val = _val;				     \
> > > -		typeof(_mask) __mask = _mask;				     \
> > > -		__BF_FIELD_CHECK(__mask, ((u16)0U), __val,		     \
> > > -				 "HWORD_UPDATE: ");			     \
> > > -		(((typeof(__mask))(__val) << __bf_shf(__mask)) & (__mask)) | \
> > > -		((__mask) << 16);					     \
> > > -	})
> > > +#define FIELD_PREP_WM16(mask, val)				\
> > > +({								\
> > > +	__auto_type _mask = mask;				\
> > > +	u32 _val = FIELD_PREP(_mask, val);			\
> > > +	BUILD_BUG_ON_MSG(_mask > 0xffffu,			\
> > > +			 "FIELD_PREP_WM16: mask too large");	\
> > > +	_val | (_mask << 16);					\
> > > +})
> > >  
> > >  /**
> > >   * FIELD_PREP_WM16_CONST() - prepare a constant bitfield element with a mask in
> > >   
> > 
> > This breaks the build for at least one driver that uses
> > FIELD_PREP_WM16, namely phy-rockchip-emmc.c:
> 
> Not in my allmodconfig build.
> ... 
> > pcie-dw-rockchip.c is similarly broken by this change, except
> > without the superfluous wrapper:
> 
> That one did get built.

I build with clang 21.1.6 for arm64, in case that's any help.
I don't see how pcie-dw-rockchip.c built for you if FIELD_PREP
and FIELD_PREP_WM16 have conflicting symbol names?

> 
> The problem is that FIELD_PREP_WM16() needs to use different 'local'
> variables than FIELD_PREP().
> The 'proper' fix is to use unique names (as min() and max() do), but that
> makes the whole thing unreadable and is best avoided unless nesting is
> likely.
> In this case s/mask/wm16_mask/ and s/val/wm16_val/ might be best.
> 
> 	David
> 
>