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Date: Fri, 16 Dec 2016 17:20:22 +0000 From: Ard Biesheuvel <ard.biesheuvel@...aro.org> To: Arnd Bergmann <arnd@...db.de> Cc: Russell King <linux@...linux.org.uk>, Nicolas Pitre <nicolas.pitre@...aro.org>, "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, "linux-arm-kernel@...ts.infradead.org" <linux-arm-kernel@...ts.infradead.org>, Jonas Jensen <jonas.jensen@...il.com> Subject: Re: [PATCH] ARM: disallow ARM_THUMB for ARMv4 builds On 16 December 2016 at 09:14, Arnd Bergmann <arnd@...db.de> wrote: > With old compilers (gcc-4.3 and earlier), we run into a build error > when CONFIG_ARM_THUMB is enabled in kernels that have both ARMv4 and > ARMv4T/ARMv5 CPU support: > > arch/arm/kernel/entry-armv.S: Assembler messages: > arch/arm/kernel/entry-armv.S:938: Error: selected processor does not support `bx lr' in ARM mode > arch/arm/kernel/entry-armv.S:961: Error: selected processor does not support `bx lr' in ARM mode > arch/arm/kernel/entry-armv.S:1004: Error: selected processor does not support `bx lr' in ARM mode > > The problem evidently is that 'bx' cannot work on the old CPUs, but > the new ones have to use it whenever returning to THUMB user space. > Can't we use the old tst lr, #1 moveq pc, lr bx lr trick? (where bx lr needs to be emitted as a plain opcode to hide it from the assembler) > This was discussed a while ago without a conclusion about what > the proper patch should be to solve it, and came again up now when I > experimented with old toolchain versions. > > This sidesteps the problem by declaring that we do not support > the configuration and instead have to disable CONFIG_ARM_THUMB > and not use THUMB user space with a kernel that supports the > FA526 CPU, which is the only one that is allowed in a multiplatform > configuration together with ARMv4T/ARMv5 anyway. This is not > a regression because the configuration never worked anyway. > The only platform affected by this is moxart, as no other ARMv4 > platforms are part of ARCH_MULTIPLATFORM. > > Cc: Jonas Jensen <jonas.jensen@...il.com> > Link: http://lkml.iu.edu/hypermail/linux/kernel/1404.1/00908.html > Signed-off-by: Arnd Bergmann <arnd@...db.de> > --- > arch/arm/mm/Kconfig | 6 +----- > 1 file changed, 1 insertion(+), 5 deletions(-) > > diff --git a/arch/arm/mm/Kconfig b/arch/arm/mm/Kconfig > index 5d4920c069f2..f7b7aa37964f 100644 > --- a/arch/arm/mm/Kconfig > +++ b/arch/arm/mm/Kconfig > @@ -655,11 +655,7 @@ config ARCH_DMA_ADDR_T_64BIT > > config ARM_THUMB > bool "Support Thumb user binaries" if !CPU_THUMBONLY > - depends on CPU_ARM720T || CPU_ARM740T || CPU_ARM920T || CPU_ARM922T || \ > - CPU_ARM925T || CPU_ARM926T || CPU_ARM940T || CPU_ARM946E || \ > - CPU_ARM1020 || CPU_ARM1020E || CPU_ARM1022 || CPU_ARM1026 || \ > - CPU_XSCALE || CPU_XSC3 || CPU_MOHAWK || CPU_V6 || CPU_V6K || \ > - CPU_V7 || CPU_FEROCEON || CPU_V7M > + depends on !(CPU_32v3 || CPU_32v4) > default y > help > Say Y if you want to include kernel support for running user space > -- > 2.9.0 > > > _______________________________________________ > linux-arm-kernel mailing list > linux-arm-kernel@...ts.infradead.org > http://lists.infradead.org/mailman/listinfo/linux-arm-kernel
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