linux-kernel - [PATCH] Re: Hardcoded instruction causes certain features to fail on ARM platfrom due to endianness

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Date:	Mon, 15 Oct 2012 23:33:08 +0800
From:	Fei Yang <yangfei.kernel@...il.com>
To:	Mikael Pettersson <mikpe@...uu.se>, linux@....linux.org.uk,
	lethal@...ux-sh.org, magnus.damm@...il.com, kgene.kim@...sung.com,
	linux-arm-kernel@...ts.infradead.org
Cc:	linux-hotplug@...r.kernel.org,
	"linux-assembly@...r.kernel.org" <linux-assembly@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Yangfei (Felix)" <felix.yang@...wei.com>
Subject: [PATCH] Re: Hardcoded instruction causes certain features to fail on
 ARM platfrom due to endianness

2012/10/15 Mikael Pettersson <mikpe@...uu.se>:
> Yangfei (Felix) writes:
>  > Hi all,
>  >
>  >     I found that hardcoded instruction in inline asm can cause certains certain features fail to work on ARM platform due to endianness.
>  >     As an example, consider the following code snippet of platform_do_lowpower function from arch/arm/mach-realview/hotplug.c:
>  >                 / *
>  >                  * here's the WFI
>  >                  */
>  >                 asm(".word      0xe320f003\n"
>  >                     :
>  >                     :
>  >                     : "memory", "cc");
>  >
>  >     The instruction generated from this inline asm will not work on big-endian ARM platform, such as ARM BE-8 format. Instead, an exception will be generated.
>  >
>  >     Here the code should be:
>  >                 / *
>  >                  * here's the WFI
>  >                  */
>  >                 asm("WFI\n"
>  >                     :
>  >                     :
>  >                     : "memory", "cc");
>  >
>  >     Seems the kernel doesn't support ARM BE-8 well. I don't know why this problem happens.
>  >     Can anyone tell me who owns this part? I can prepare a patch then.
>  >     Thanks.
>
> Questions regarding the ARM kernel should go to the linux-arm-kernel mailing list
> (see the MAINTAINERS file), with an optional cc: to the regular LKML.
>
> BE-8 is, if I recall correctly, ARMv7's broken format where code and data have
> different endianess.  GAS supports an ".inst" directive which is like ".word"
> except the data is assumed to be code.  This matters for disassembly, and may
> also be required for BE-8.
>
> That is, just s/.word/.inst/g above and report back if that works or not.
> --
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> Please read the FAQ at  http://www.tux.org/lkml/
>
>

Hi Mikael,

Thanks for the reply. I modified the code as suggested and rebuilt the
kernel, cpu-hotplug feature now works on big-endian(BE-8) ARM
platform.
Since the ARM core can be configured by system software to work in
big-endian mode, it's necessary to fix this problem. And here is a
small patch :

diff -urN linux-3.6.2/arch/arm/mach-exynos/hotplug.c
linux/arch/arm/mach-exynos/hotplug.c
--- linux-3.6.2/arch/arm/mach-exynos/hotplug.c	2012-10-13
04:50:59.000000000 +0800
+++ linux/arch/arm/mach-exynos/hotplug.c	2012-10-15 23:05:44.000000000 +0800
@@ -72,7 +72,7 @@
 		/*
 		 * here's the WFI
 		 */
-		asm(".word	0xe320f003\n"
+		asm(".inst	0xe320f003\n"
 		    :
 		    :
 		    : "memory", "cc");
diff -urN linux-3.6.2/arch/arm/mach-realview/hotplug.c
linux/arch/arm/mach-realview/hotplug.c
--- linux-3.6.2/arch/arm/mach-realview/hotplug.c	2012-10-13
04:50:59.000000000 +0800
+++ linux/arch/arm/mach-realview/hotplug.c	2012-10-15 23:05:00.000000000 +0800
@@ -66,7 +66,7 @@
 		/*
 		 * here's the WFI
 		 */
-		asm(".word	0xe320f003\n"
+		asm(".inst	0xe320f003\n"
 		    :
 		    :
 		    : "memory", "cc");
diff -urN linux-3.6.2/arch/arm/mach-shmobile/hotplug.c
linux/arch/arm/mach-shmobile/hotplug.c
--- linux-3.6.2/arch/arm/mach-shmobile/hotplug.c	2012-10-13
04:50:59.000000000 +0800
+++ linux/arch/arm/mach-shmobile/hotplug.c	2012-10-15 23:05:25.000000000 +0800
@@ -53,7 +53,7 @@
 		/*
 		 * here's the WFI
 		 */
-		asm(".word	0xe320f003\n"
+		asm(".inst	0xe320f003\n"
 		    :
 		    :
 		    : "memory", "cc");
--
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