[U-Boot] [PATCH 1/2] sunxi: Use Thumb2 and move stack to gain more SRAM space in FEL mode

[Siarhei Siamashka]

The Allwinner SoCs support a special FEL boot mode, which can be activated
by users via a button press (or other means). In the FEL mode, the BROM
implements a custom FEL protocol over USB, which allows to upload code to
the device and run it. This protocol had been reverse engineered and
documented by Henrik Nordström:

    http://lists.phcomp.co.uk/pipermail/arm-netbook/2012-June/004341.html

Because the BROM code is using some parts of the SRAM for itself, only a
few areas are available for use in u-boot. Currently the SPL is loaded
into the "0x2000-0x5cff Free for program use" area and the stack pointer
is at the end of this area. This is barely enough to fit just the current
SPL and leaves almost no headroom for the future code.

This patch enables the use of a more compact Thumb2 mode for compiling the
FEL SPL binary. And also relocates the stack to another "0x8000-0xbfff Free
for program use" area. Additionally, the BSS segment is cleared.

Signed-off-by: Siarhei Siamashka <siarhei.siamashka at gmail.com>
---
 arch/arm/cpu/armv7/sunxi/Makefile           |  1 +
 arch/arm/cpu/armv7/sunxi/start_fel.S        | 42 +++++++++++++++++++++++++++++
 arch/arm/cpu/armv7/sunxi/u-boot-spl-fel.lds |  4 +--
 include/configs/sunxi-common.h              |  2 --
 4 files changed, 45 insertions(+), 4 deletions(-)
 create mode 100644 arch/arm/cpu/armv7/sunxi/start_fel.S

diff --git a/arch/arm/cpu/armv7/sunxi/Makefile b/arch/arm/cpu/armv7/sunxi/Makefile
index a64bfa1..b3eff98 100644
--- a/arch/arm/cpu/armv7/sunxi/Makefile
+++ b/arch/arm/cpu/armv7/sunxi/Makefile
@@ -21,5 +21,6 @@ ifdef CONFIG_SPL_BUILD
 obj-$(CONFIG_SUN7I)	+= dram.o
 ifdef CONFIG_SPL_FEL
 obj-y	+= start.o
+extra-y += start_fel.o
 endif
 endif
diff --git a/arch/arm/cpu/armv7/sunxi/start_fel.S b/arch/arm/cpu/armv7/sunxi/start_fel.S
new file mode 100644
index 0000000..2789fd9
--- /dev/null
+++ b/arch/arm/cpu/armv7/sunxi/start_fel.S
@@ -0,0 +1,42 @@
+/*
+ * Copyright (c) 2014 Siarhei Siamashka <siarhei.siamashka at gmail.com>
+ *
+ * SPDX-License-Identifier:	GPL-2.0+
+ */
+
+.syntax unified
+.text
+.arm
+.arch armv7a
+.p2align 2
+
+.globl _start_fel
+.globl s_init
+.globl __bss_start
+.globl __bss_end
+
+_start_fel:
+	/* Relocate stack to the 0x8000-0xBFFF area */
+	mov	r0, #0xC000
+	str	sp, [r0, #-4]!
+	str	lr, [r0, #-4]!
+	adr	lr, _exit_fel /* Return back to '_exit_fel' */
+	mov	sp, r0
+
+	/* Erase the BSS segment */
+	ldr	r0, =__bss_start
+	ldr	r1, =__bss_end
+	mov	r2, #0
+0:	cmp	r0, r1
+	strbne	r2, [r0], #1
+	bne	0b
+
+	/* Pass control to the 's_init()' function */
+	b	s_init
+
+_exit_fel:
+	/* Relocate stack back and return */
+	mov	r0, #0xC000
+	ldr	sp, [r0, #-4]!
+	ldr	lr, [r0, #-4]!
+	bx	lr
diff --git a/arch/arm/cpu/armv7/sunxi/u-boot-spl-fel.lds b/arch/arm/cpu/armv7/sunxi/u-boot-spl-fel.lds
index 364e35c..418c2fc 100644
--- a/arch/arm/cpu/armv7/sunxi/u-boot-spl-fel.lds
+++ b/arch/arm/cpu/armv7/sunxi/u-boot-spl-fel.lds
@@ -6,7 +6,7 @@
  */
 OUTPUT_FORMAT("elf32-littlearm", "elf32-littlearm", "elf32-littlearm")
 OUTPUT_ARCH(arm)
-ENTRY(s_init)
+ENTRY(_start_fel)
 SECTIONS
 {
 	. = 0x00002000;
@@ -14,7 +14,7 @@ SECTIONS
 	. = ALIGN(4);
 	.text :
 	{
-		*(.text.s_init)
+		arch/arm/cpu/armv7/sunxi/start_fel.o	(.text)
 		*(.text*)
 	}
 
diff --git a/include/configs/sunxi-common.h b/include/configs/sunxi-common.h
index 5d72d62..4b980e9 100644
--- a/include/configs/sunxi-common.h
+++ b/include/configs/sunxi-common.h
@@ -18,10 +18,8 @@
  */
 #define CONFIG_SUNXI		/* sunxi family */
 #ifdef CONFIG_SPL_BUILD
-#ifndef CONFIG_SPL_FEL
 #define CONFIG_SYS_THUMB_BUILD	/* Thumbs mode to save space in SPL */
 #endif
-#endif
 
 #include <asm/arch/cpu.h>	/* get chip and board defs */
 
-- 
1.8.3.2