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

[Aneesh V]

Loic,

On Monday 14 March 2011 09:41 PM, Loïc Minier wrote:
> 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.

I will do that. Thank you.

>
>> @@ -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).
>

Please note that I am enabling armv7-a in the second patch in omap4
config.mk file. The reason I didn't do this here was some ARMv7 SoCs do
not want to use -march=armv7-a even if the compiler supports it. Tegra2
is an example. Please see the below from Tegra2 config.mk:

# Use ARMv4 for Tegra2 - initial code runs on the AVP, which is an ARM7TDI.
PLATFORM_CPPFLAGS += -march=armv4

This being the case I would have had to define another CONFIG flag if I
had to add -march=armv7-a in arch/arm/cpu/armv7/config.mk. I thought it
un-necessary and instead put it in the SoC specific file. So, Tegra2
can continue to use -march=armv4 and will get Thumb-1 if they enable
CONFIG_SYS_THUMB_BUILD. Or do you think we should define something like
CONFIG_SYS_MARCH_ARMV7

I will try -march=armv7, but I vaguely remember it had some issues.

>>   # =========================================================================
>>   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
>>
>