[U-Boot] [RFC PATCH 0/4] Enable Thumb build for ARM platforms

[Aneesh V]

Thumb is an alternate instruction set available in many
ARM processors. Below is a detailed description from ARM
specs:

"The Thumb instruction set is a re-encoded subset of the
ARM instruction set. Thumb instructions execute in their
own processor state, with the architecture defining the
mechanisms required to transition between ARM and Thumb
states. The key difference is that Thumb instructions are
half the size of ARM instructions(16 bits compared with 32
bits). Greater code density can usually be achieved by using
the Thumb instruction set in preference to the ARM instruction
set, at a cost of some reduction in performance"

"In ARMv6T2, Thumb-2 technology is introduced. This technology
makes it possible to extend the original Thumb instruction set
with many 32-bit instructions. The range of 32-bit Thumb instructions
included in ARMv6T2 permits Thumb code to achieve performance
similar to ARM code, with code density better than that of earlier
Thumb code. From ARMv6T2, the ARM and Thumb instruction sets provide
almost identical functionality"

This series adds Thumb support in U-Boot and enables it for
OMAP4. It also fixes issues faced while booting OMAP4 with
Thumb-2 images of U-Boot and SPL.

Thumb mode is becoming increasingly relevant for U-Boot with
the advent of SPL. It's very important to keep SPL size smaller
considering the internal RAM size constraints on many platforms.
On OMAP4 the size reduction enables us to use SPL on secure devices
that have smaller internal RAM available for non-secure world. 

I would request all who are interested in this feature to test it
and give feedback. To make that easier I have pushed my patches
here (along with the timer patch from Nicolas that fixes boot on
OMAP4): 

git at github.com:aneeshv/u-boot.git
branch: thumb

To enable support for new platforms you just need to add
CONFIG_SYS_THUMB_BUILD in your config file.

Aneesh V (4):
  ARM: enable Thumb build
  OMAP3+: fix issues with Thumb build
  OMAP3+: Use -march=armv7-a and thereby enable Thumb-2
  OMAP4: enable Thumb build

 README                                         |    9 +++++++
 arch/arm/config.mk                             |   29 ++++++++++++++----------
 arch/arm/cpu/armv7/cpu.c                       |    4 ++-
 arch/arm/cpu/armv7/omap-common/config.mk       |    1 -
 arch/arm/cpu/armv7/omap-common/hwinit-common.c |   25 ++++++++++++++++++++
 arch/arm/cpu/armv7/omap-common/lowlevel_init.S |    4 +-
 arch/arm/cpu/armv7/omap3/config.mk             |    2 +
 arch/arm/cpu/armv7/omap4/config.mk             |    2 +
 include/configs/omap4_common.h                 |    2 +
 9 files changed, 62 insertions(+), 16 deletions(-)