Date: Mon, 2 Feb 2015 15:18:24 -0800
From: Andrew Morton <akpm@linux-foundation.org>
To: Russell King - ARM Linux <linux@arm.linux.org.uk>
Cc: Christoph Jaeger <cj@linux.com>, linux-kernel@vger.kernel.org,
        Yalin Wang <yalin.wang@sonymobile.com>,
        Will Deacon <will.deacon@arm.com>
Subject: Re: [PATCH linux-next] lib: Kconfig: use bool instead of boolean
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	<20150202230548.GJ8656@n2100.arm.linux.org.uk>
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On Mon, 2 Feb 2015 23:05:48 +0000 Russell King - ARM Linux <linux@arm.linux.org.uk> wrote:

> On Mon, Feb 02, 2015 at 02:27:32PM -0800, Andrew Morton wrote:
> > On Mon,  2 Feb 2015 09:59:16 -0500 Christoph Jaeger <cj@linux.com> wrote:
> > 
> > > Keyword 'boolean' for type definition attributes is considered
> > > deprecated and, therefore, should not be used anymore.
> > > 
> > > See http://lkml.kernel.org/r/cover.1418003065.git.cj@linux.com
> > > See http://lkml.kernel.org/r/1419108071-11607-1-git-send-email-cj@linux.com
> > > 
> > > ...
> > >
> > > --- a/lib/Kconfig
> > > +++ b/lib/Kconfig
> > > @@ -14,7 +14,7 @@ config BITREVERSE
> > >  	tristate
> > >  
> > >  config HAVE_ARCH_BITREVERSE
> > > -	boolean
> > > +	bool
> > >  	default n
> > >  	depends on BITREVERSE
> > >  	help
> > 
> > Your patch patches 556d2f055bf6d ("ARM: 8187/1: add
> > CONFIG_HAVE_ARCH_BITREVERSE to support rbit instruction") which appears
> > in linux-next via the ARM tree.
> > 
> > There are many uses of "boolean" in lib/Kconfig.  Converting just one
> > of them is inefficient and odd.
> > 
> > 556d2f055bf6d is a bit of a surprise.  It looks good to me from a
> > non-ARM perspective - the __builtin_constant_p() optimisation is
> > sensible, although bitrev on a constant probably isn't very common.
> > 
> > I'm not sure about the ARM part though!  __bitrev8() is pretty damn
> > fast.  Presumably an inlined rbit instruction is faster still, but not
> > very much?
> > 
> > The Kconfig help text in 556d2f055bf6d rather needs some caring for.
> 
> The patches had already been round six iterations, and had been posted
> on LKML for every iteration.

People saw "ARM" and went Zzzzz ;)

I'm a bit surprised that nobody helped out with the Kconfig text. 
I queued the below.  Looks OK?

--- a/lib/Kconfig~a
+++ a/lib/Kconfig
@@ -18,9 +18,8 @@ config HAVE_ARCH_BITREVERSE
 	default n
 	depends on BITREVERSE
 	help
-	  This option provides an config for the architecture which have instruction
-	  can do bitreverse operation, we use the hardware instruction if the architecture
-	  have this capability.
+	  This option enables the use of hardware bit-reversal instructions on
+	  architectures which support such operations.
 
 config RATIONAL
 	bool
_


> When people started pushing to have the patches merged, there were two
> dependent patches which had been merged via other random trees, so I
> held them off for a cycle.  At that point, I even questioned whether I
> should be merging them; that question was ignored by everyone.
> 
> Eventually, (and after some testing) I ended up giving up and merging
> them because they're believed to be a net benefit for ARM,

I know the feeling ;)

> and I
> couldn't locate anyone who'd be useful to ack the generic parts of the
> patch.

I usually look after lib/ and hereby ack the patch!  I really should do
a MAINTAINERS patch but I'm shy.

> As for whether __bitrev8 is fast or not, that depends whether the table
> has been speculatively prefetched and is available without having to go
> out to memory - and it's not just about the table itself, there's also
> the loading from the individual function's literal pool to get the
> address of the table too.  So that's two memory loads per rbit at
> minimum.
> 
> The rbit instruction is probably at least half the average cycles of
> two dependent loads.

OK.
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