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Date:   Thu, 21 Dec 2017 17:57:17 +0100
From:   Linus Walleij <linus.walleij@...aro.org>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Russell King <linux@...linux.org.uk>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH, RFT] ARM: use --fix-v4bx to allow building ARMv4 with
 future gcc

On Wed, Dec 20, 2017 at 2:00 PM, Arnd Bergmann <arnd@...db.de> wrote:

> gcc-6.0 and later marks support for ARMv3 and ARMv4 as 'deprecated',
> meaning that this is expected to be removed at some point in the future,
> with gcc-8.0 as the earliest.
>
> When building the kernel, the difference between ARMv4 and ARMv4T
> is relatively small because the kernel never runs THUMB instructions
> on ARMv4T and does not need any support for interworking.
>
> For any future compiler that does not support -march=armv4, we now
> fall back to -march=armv4t as the architecture level selection,
> but keep using -march=armv4 by default as long as that is supported
> by the compiler.
>
> Similarly, the -mtune=strongarm110 and -mtune=strongarm1100 options
> will go away at the same time as -march=armv4, so this adds a check
> to see if the compiler supports them, falling back to no -mtune
> option otherwise.
>
> Compiling with -march=armv4t leads the compiler to using 'bx reg'
> instructions instead of 'mov pc,reg'. This is not supported on
> ARMv4 based CPUs, but the linker can work around this by rewriting
> those instructions to the ARMv4 version if we pass --fix-v4bx
> to the linker. This should work with binutils-2.15 (released
> May 2004) or higher, and we can probably assume that anyone using
> gcc-7.x will have a much more recent binutils version as well.
>
> However, in order to still allow users of old toolchains to link
> the kernel, we only pass the option to linkers that support it,
> based on a $(ld-option ...) call. I'm intentionally passing the
> flag to all linker versions here regardless of whether it's needed
> or not, so we can more easily spot any regressions if something
> goes wrong.
>
> For consistency, I'm passing the --fix-v4bx flag for both the
> vmlinux final link and the individual loadable modules.
> The module loader code already interprets the R_ARM_V4BX relocations
> in loadable modules and converts bx instructions into mov even
> when running on ARMv4T or ARMv5 processors. This is now redundant
> when we pass --fix-v4bx to the linker for building modules, but
> I see no harm in leaving the current implementation and doing both.
>
> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> ---
> Please test by making the -march=armv4t switch unconditional
> and see if that results in a working kernel

I did this:
diff --git a/arch/arm/Makefile b/arch/arm/Makefile
index 66e46aec0cd0..3944ecd6cd31 100644
--- a/arch/arm/Makefile
+++ b/arch/arm/Makefile
@@ -81,7 +81,7 @@ arch-$(CONFIG_CPU_32v6K)
=-D__LINUX_ARM_ARCH__=6 $(call cc-option,-march=armv6k,
 endif
 arch-$(CONFIG_CPU_32v5)                =-D__LINUX_ARM_ARCH__=5 $(call
cc-option,-march=armv5te,-march=armv4t)
 arch-$(CONFIG_CPU_32v4T)       =-D__LINUX_ARM_ARCH__=4 -march=armv4t
-arch-$(CONFIG_CPU_32v4)                =-D__LINUX_ARM_ARCH__=4 $(call
cc-option,-march=armv4,-march=armv4t)
+arch-$(CONFIG_CPU_32v4)                =-D__LINUX_ARM_ARCH__=4 -march=armv4t
 arch-$(CONFIG_CPU_32v3)                =-D__LINUX_ARM_ARCH__=3 -march=armv3

Built and booted on the Gemini platform.

It crashes immediately and goes into the boot loader
on thos FA-526 based platform.

Yours,
Linus Walleij

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