netdev - Re: [PATCH v2 08/16] bitfield: Simplify __BF_FIELD_CHECK

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Message-ID: <20251228225004.69a6972f@pumpkin>
Date: Sun, 28 Dec 2025 22:50:04 +0000
From: David Laight <david.laight.linux@...il.com>
To: Andy Shevchenko <andriy.shevchenko@...el.com>
Cc: Jonathan Cameron <jonathan.cameron@...wei.com>, 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>, 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>, 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 v2 08/16] bitfield: Simplify __BF_FIELD_CHECK_REG()

On Sun, 28 Dec 2025 20:53:45 +0200
Andy Shevchenko <andriy.shevchenko@...el.com> wrote:

> On Wed, Dec 17, 2025 at 10:31:55PM +0000, David Laight wrote:
> > On Wed, 17 Dec 2025 10:26:18 +0000
> > Jonathan Cameron <jonathan.cameron@...wei.com> wrote:  
> > > On Fri, 12 Dec 2025 19:37:13 +0000
> > > david.laight.linux@...il.com wrote:  
> 
> ...
> 
> > > > +	BUILD_BUG_ON_MSG((mask) + 0U + 0UL + 0ULL >			\
> > > > +			 ~0ULL >> (64 - 8 * sizeof (reg)),		\    
> > > 
> > > Trivial.  sizeof(reg) is much more comment syntax in kernel code.  
> >                                      (common)
> > 
> > Hmm. sizeof is an operator not a function.
> > Its argument is either a variable/expression or a bracketed type
> > (I don't usually put variables in brackets).
> > So 'sizeof(reg)' is nearly as bad as 'return(reg)'.  
> 
> Yet, it's a style used de facto in the Linux kernel. I am with Jonathan on this.

Not hard to change :-)

There is a more interesting question as to whether that check is
needed at all?
It is only really the sanity checks (and the fact that __builtin_ffs() is
sub-optimal) that stop the functions being 'reasonable' for non-constant
values.

	David