linux-kernel - Re: [RFC V6 2/3] arm:add bitrev.h file to support rbit instruction

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Message-ID: <1415948109.5912.12.camel@perches.com>
Date:	Thu, 13 Nov 2014 22:55:09 -0800
From:	Joe Perches <joe@...ches.com>
To:	Takashi Iwai <tiwai@...e.de>
Cc:	Russell King - ARM Linux <linux@....linux.org.uk>,
	"Wang, Yalin" <Yalin.Wang@...ymobile.com>,
	'Will Deacon' <will.deacon@....com>,
	'Ard Biesheuvel' <ard.biesheuvel@...aro.org>,
	"'linux-kernel@...r.kernel.org'" <linux-kernel@...r.kernel.org>,
	"'akinobu.mita@...il.com'" <akinobu.mita@...il.com>,
	"'linux-mm@...ck.org'" <linux-mm@...ck.org>,
	"'linux-arm-kernel@...ts.infradead.org'" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [RFC V6 2/3] arm:add bitrev.h file to support rbit instruction

On Fri, 2014-11-14 at 07:37 +0100, Takashi Iwai wrote:
> At Thu, 13 Nov 2014 16:05:30 -0800,
> Joe Perches wrote:
> > 
> > On Thu, 2014-11-13 at 23:53 +0000, Russell King - ARM Linux wrote:
> > > On Fri, Oct 31, 2014 at 01:42:44PM +0800, Wang, Yalin wrote:
> > > > This patch add bitrev.h file to support rbit instruction,
> > > > so that we can do bitrev operation by hardware.
> > > > Signed-off-by: Yalin Wang <yalin.wang@...ymobile.com>
> > > > ---
> > > >  arch/arm/Kconfig              |  1 +
> > > >  arch/arm/include/asm/bitrev.h | 21 +++++++++++++++++++++
> > > >  2 files changed, 22 insertions(+)
> > > >  create mode 100644 arch/arm/include/asm/bitrev.h
> > > > 
> > > > diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
> > > > index 89c4b5c..be92b3b 100644
> > > > --- a/arch/arm/Kconfig
> > > > +++ b/arch/arm/Kconfig
> > > > @@ -28,6 +28,7 @@ config ARM
> > > >  	select HANDLE_DOMAIN_IRQ
> > > >  	select HARDIRQS_SW_RESEND
> > > >  	select HAVE_ARCH_AUDITSYSCALL if (AEABI && !OABI_COMPAT)
> > > > +	select HAVE_ARCH_BITREVERSE if (CPU_V7M || CPU_V7)
> > > 
> > > Looking at this, this is just wrong.  Take a moment to consider what
> > > happens if we build a kernel which supports both ARMv6 _and_ ARMv7 CPUs.
> > > What happens if an ARMv6 CPU tries to execute an rbit instruction?
> > > 
> > > Second point (which isn't obvious from your submissions on-list) is that
> > > you've loaded the patch system up with patches for other parts of the
> > > kernel tree for which I am not responsible for.  As such, I can't take
> > > those patches without the sub-tree maintainer acking them.  Also, the
> > > commit text in those patches look weird:
> > > 
> > > 6fire: Convert byte_rev_table uses to bitrev8
> > > 
> > > Use the inline function instead of directly indexing the array.
> > > 
> > > This allows some architectures with hardware instructions for bit
> > > reversals to eliminate the array.
> > > 
> > > Signed-off-by: Joe Perches <(address hidden)>
> > > Signed-off-by: Yalin Wang <(address hidden)>
> > > 
> > > Why is Joe signing off on these patches?
> > > Shouldn't his entry be an Acked-by: ?
> > 
> > I didn't sign off on or ack the "add bitrev.h" patch.
> > 
> > I created 2 patches that converted direct uses of byte_rev_table
> > to that bitrev8 static inline.  One of them is already in -next
> > 
> > 7a1283d8f5298437a454ec477384dcd9f9f88bac carl9170: Convert byte_rev_table uses to bitrev8
> > 
> > The other hasn't been applied.
> > 
> > https://lkml.org/lkml/2014/10/28/1056
> > 
> > Maybe Takashi Iwai will get around to it one day.
> 
> It was not clear to me whether I should apply it individually from
> others in the whole thread.  Your description looked as if it makes
> sense only with ARM's bitrev8 support.
> 
> So, again: should I apply this now to my tree?

I it would be good to apply even if the
bitrev patch for arm is never applied.

$ git grep -w bitrev8 | wc -l
110

vs

this last direct use of byte_rev_table.

cheers, Joe

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