linux-kernel - Re: [RFC PATCH 2/2] treewide, bits: use ffs

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Message-ID: <alpine.DEB.2.21.2601100413140.30566@angie.orcam.me.uk>
Date: Sat, 10 Jan 2026 10:36:07 +0000 (GMT)
From: "Maciej W. Rozycki" <macro@...am.me.uk>
To: Yury Norov <ynorov@...dia.com>
cc: Petr Tesarik <ptesarik@...e.com>, Yury Norov <yury.norov@...il.com>, 
    Rasmus Villemoes <linux@...musvillemoes.dk>, 
    Richard Henderson <richard.henderson@...aro.org>, 
    Matt Turner <mattst88@...il.com>, Magnus Lindholm <linmag7@...il.com>, 
    Vineet Gupta <vgupta@...nel.org>, 
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    Vasily Gorbik <gor@...ux.ibm.com>, 
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    Pablo Neira Ayuso <pablo@...filter.org>, Florian Westphal <fw@...len.de>, 
    linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 2/2] treewide, bits: use ffs_val() where it is
 open-coded

On Fri, 9 Jan 2026, Yury Norov wrote:

> > diff --git a/arch/alpha/kernel/smp.c b/arch/alpha/kernel/smp.c
> > index ed06367ece574..2f3809c015b99 100644
> > --- a/arch/alpha/kernel/smp.c
> > +++ b/arch/alpha/kernel/smp.c
> > @@ -525,7 +525,7 @@ handle_ipi(struct pt_regs *regs)
> >  	  do {
> >  		unsigned long which;
> >  
> > -		which = ops & -ops;
> > +		which = ffs_val(ops);
> >  		ops &= ~which;
> >  		which = __ffs(which);
> 
> I guess, this should be:
> 
>         which = __ffs(ops);
>         ops &= ops - 1;

 I think it's the wrong operation order.

> > diff --git a/arch/mips/dec/ecc-berr.c b/arch/mips/dec/ecc-berr.c
> > index 1eb356fdd8323..8934b8b1cf375 100644
> > --- a/arch/mips/dec/ecc-berr.c
> > +++ b/arch/mips/dec/ecc-berr.c
> > @@ -153,7 +153,7 @@ static int dec_ecc_be_backend(struct pt_regs *regs, int is_fixup, int invoker)
> >  			/* Ack now, now we've rewritten (or not). */
> >  			dec_ecc_be_ack();
> >  
> > -			if (syn && syn == (syn & -syn)) {
> > +			if (syn && syn == ffs_val(syn)) {
> 
> It opencodes is_power_of_2().

 Correct and it also predates that helper's existence by ~4 years.  I'll 
be happy to see this converted as the intent will be obviously clearer.

  Maciej