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

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Message-ID: <20260110115439.184b7a66@pumpkin>
Date: Sat, 10 Jan 2026 11:54:39 +0000
From: David Laight <david.laight.linux@...il.com>
To: "Maciej W. Rozycki" <macro@...am.me.uk>
Cc: Yury Norov <ynorov@...dia.com>, 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>, Geert Uytterhoeven <geert@...ux-m68k.org>, Thomas
 Bogendoerfer <tsbogend@...ha.franken.de>, Madhavan Srinivasan
 <maddy@...ux.ibm.com>, Michael Ellerman <mpe@...erman.id.au>, Heiko
 Carstens <hca@...ux.ibm.com>, Vasily Gorbik <gor@...ux.ibm.com>, Alexander
 Gordeev <agordeev@...ux.ibm.com>, Chris Zankel <chris@...kel.net>, Max
 Filippov <jcmvbkbc@...il.com>, Patrik Jakobsson
 <patrik.r.jakobsson@...il.com>, Maarten Lankhorst
 <maarten.lankhorst@...ux.intel.com>, Maxime Ripard <mripard@...nel.org>,
 Thomas Zimmermann <tzimmermann@...e.de>, David Airlie <airlied@...il.com>,
 Simona Vetter <simona@...ll.ch>, Robin Murphy <robin.murphy@....com>, Joerg
 Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>, Jakub Kicinski
 <kuba@...nel.org>, Andrew Lunn <andrew+netdev@...n.ch>, "David S. Miller"
 <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, Paolo Abeni
 <pabeni@...hat.com>, Oliver Neukum <oliver@...kum.org>, Arnd Bergmann
 <arnd@...db.de>, Kuan-Wei Chiu <visitorckw@...il.com>, Andrew Morton
 <akpm@...ux-foundation.org>, Marcel Holtmann <marcel@...tmann.org>, Johan
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 <luiz.dentz@...il.com>, 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 Sat, 10 Jan 2026 10:36:07 +0000 (GMT)
"Maciej W. Rozycki" <macro@...am.me.uk> wrote:

> 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.

If you are going to do the __ffs() do it first.
		which = __ffs(ops);
		ops &= ~(1u << which);

OTOH what follows is:
		switch (which) {
		case IPI_RESCHEDULE:
so changing to 'case 1u << IPI_RESCHEDULE: would save the ffs().
But the entire loop is pointless, three tests, the first being:
	if (op & (1u << IPI_RESCHEDULE))
		scheduler_ipi();
would be far less code.

But probably pointless churn - no one cares about alpha any more :-)

> 
> > > 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().

Badly...

	David

> 
>  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
>