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

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Message-ID: <20251209185452.707f4dbe@pumpkin>
Date: Tue, 9 Dec 2025 18:54:52 +0000
From: David Laight <david.laight.linux@...il.com>
To: Andy Shevchenko <andriy.shevchenko@...el.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>, 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>,
 Nicolas Frattaroli <nicolas.frattaroli@...labora.com>
Subject: Re: [PATCH 3/9] bitmap: Use FIELD_PREP() in expansion of
 FIELD_PREP_WM16()

On Tue, 9 Dec 2025 17:46:21 +0200
Andy Shevchenko <andriy.shevchenko@...el.com> wrote:

> On Tue, Dec 09, 2025 at 10:03:07AM +0000, david.laight.linux@...il.com wrote:
> 
> > 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.  
> 
> ...
> 
> > +#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");	\  
> 
> Can it be static_assert() instead?

No, they have to be 'integer constant expressions' not just
'compile time constants'.
Pretty useless for anything non-trivial.

	David

> 
> > +	_val | (_mask << 16);					\
> > +})  
>