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

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Message-ID: <20141113235322.GC4042@n2100.arm.linux.org.uk>
Date:	Thu, 13 Nov 2014 23:53:22 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	"Wang, Yalin" <Yalin.Wang@...ymobile.com>
Cc:	'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>,
	'Joe Perches' <joe@...ches.com>,
	"'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, 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?  As his is the first sign-off,
one assumes that he was responsible for creating the patch in the first
place, but there is no From: line marking him as the author.  Shouldn't
his entry be an Acked-by: ?

Confused.

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