[U-Boot] [RFC PATCH 1/2] armv7: enable Thumb build for armv7

[Loïc Minier]

On Mon, Mar 14, 2011, Aneesh V wrote:
> +- ARM Options:
> +		CONFIG_SYS_THUMB_BUILD
> +
> +		Use this flag to build U-Boot using the Thumb instruction
> +		set for ARM architectures. Thumb instruction set provides
> +		better code density. For ARM architectures that support
> +		Thumb2 this flag will result in Thumb2 code generated by
> +		GCC.
> +
>  - Linux Kernel Interface:
>  		CONFIG_CLOCKS_IN_MHZ
>  

 The above README changes suggest that "Thumb-1" would also be possible
 but you patch an armv7 file:

> --- a/arch/arm/cpu/armv7/config.mk
> +++ b/arch/arm/cpu/armv7/config.mk

 so I suspect only armv7 CPUs would be affected?  Consider patching
 arch/arm/config.mk instead which is where -marm is currently set:
    # Explicitly specifiy 32-bit ARM ISA since toolchain default can be -mthumb:
    PLATFORM_CPPFLAGS += $(call cc-option,-marm,)
 In fact, I would find it less confusing if the flags were -mthumb or
 -marm and never -marm -mthumb, so you probably want to patch the code
 setting -marm above to be in your "else" clause.

> @@ -31,3 +31,8 @@ PLATFORM_CPPFLAGS += -march=armv5

 This suggests that u-boot is actually built in ARMv5 mode, which means
 we're missing out actual Thumb-2 instructions (v6+).

 Perhaps arch/arm/cpu/armv7/config.mk should also $(call
 cc-option,-march=armv7).  (I've never used -march=armv7 but it seems to
 exist and is probably more correct than -march=armv7-a).

>  # =========================================================================
>  PLATFORM_RELFLAGS +=$(call cc-option,-mshort-load-bytes,\
>  		    $(call cc-option,-malignment-traps,))
> +
> +ifeq ($(CONFIG_SYS_THUMB_BUILD),y)
> +# Enable Thumb mode build if compiler supports it
> +PLATFORM_CPPFLAGS += $(call cc-option,-mthumb -mthumb-interwork)
> +endif
> -- 
> 1.7.0.4
> 

-- 
Loïc Minier