[U-Boot] [PATCH v2] arm: support Thumb-1 with CONFIG_SYS_THUMB_BUILD

[Albert ARIBAUD <albert.u.boot@aribaud.net>]

Hello Albert,

On Tue, 24 Feb 2015 14:53:36 +0100, Albert ARIBAUD
<albert.u.boot@aribaud.net> wrote:
> When building a THumb-1-only target with CONFIG_SYS_THUMB_BUILD,
> some files fail to build, most of the time because they include
> mcr instructions, which only exist for Thumb-2.
> 
> Thos patch introduces a Kconfig option CONFIG_THUMB2 and uses
> it to select between Thumb-2 and ARM mode for the aforementioned
> files.
> 
> Signed-off-by: Albert ARIBAUD <albert.u.boot@aribaud.net>
> ---
> This patch has been build-tested and run-tested on ED Mini V2,
> above the "edmini: switch to SPL" patch, and found to reduce
> U-Boot size by 25% and SPL size by 14%... and to run fine. :)
> 
> This patch has also been tested against side effects on the
> non-Thumb wireless_space target. The binaries produced with
> and without ths patch were found to differ only by their
> version string.
> 
> Changes in v2:
> - fixed a typo in the commit message
> - added file arch/arm/thumb1/include/asm/proc-armv/system.h,
>   which overrides arch/arm/include/asm/proc-armv/system.h
>   when building for Thumb-1 and provides non-functional but
>   Thumb-compilable IRQ and FIQ related macros and functions.

Ok, this does not fare as good as I have hoped, because there are also
thumb1-unfrendly macros in arch/arm/include/asm/cache.h, this time with
mcr and mrc instructions.

/me hates macros used as inline functions. :(

I cannot just replace the macros with empty inline functions as I did
with arch/arm/include/asm/proc-armv/system.h, since this would cause
cache to not work. :(

This leaves only one solution: when buildding for thumb1, replace the
macros in cache.h with plain function prototypes, and provide simple
ARM-state implementations for these. We'll lose a bit of performance as
instead of a single mcr or mrc instruction, we'll have a call switching
from Thumb to ARM state, the mcr/mrc instruction, and a return to Thumb
state. Hopefully that won't hurt performance too much.

V3 in the works.

Amicalement,
-- 
Albert.