[U-Boot] [RFC][PATCH 08/21] [x86] Add RAM bootstrap functionality

[Graeme Russ]

Add a parameter to the 32-bit entry to indicate if entry is from Real
Mode or not. If entry is from Real Mode, execute the destructive 'sizer'
routine to determine memory size as we are booting cold and running in
Flash. If not entering from Real Mode, we are executing a U-Boot image
from RAM and therefore the memory size is already known (and running
'sizer' will destroy the running image)

There are now two 32-bit entry points. The first is the 'in RAM' entry
point which exists at the start of the U-Boot binary image. As such,
you can load u-boot.bin in RAM and jump directly to the load address
without needing to calculate any offsets. The second entry point is
used by the real-to-protected mode switch

This patch also changes TEXT_BASE to 0x6000000 (in RAM). You can load
the resulting image at 0x6000000 and simple go 0x6000000 from the u-boot
prompt

Hopefully a later patch will completely elliminate any dependency on
TEXT_BASE like a relocatable linux kernel (perfect world)
---
 board/eNET/config.mk  |    2 +-
 board/eNET/u-boot.lds |    9 ++++-----
 cpu/i386/start.S      |   34 ++++++++++++++++++++++++++++++++++
 cpu/i386/start16.S    |    7 +++++--
 4 files changed, 44 insertions(+), 8 deletions(-)

diff --git a/board/eNET/config.mk b/board/eNET/config.mk
index 5c64804..cb2e24d 100644
--- a/board/eNET/config.mk
+++ b/board/eNET/config.mk
@@ -21,7 +21,7 @@
 # MA 02111-1307 USA
 #
 
-TEXT_BASE = 0x38040000
+TEXT_BASE = 0x06000000
 CFLAGS_dlmalloc.o += -Wa,--no-warn -fno-strict-aliasing
 PLATFORM_RELFLAGS += -fvisibility=hidden
 PLATFORM_CPPFLAGS += -fno-dwarf2-cfi-asm
diff --git a/board/eNET/u-boot.lds b/board/eNET/u-boot.lds
index 0d74021..7b0ffaa 100644
--- a/board/eNET/u-boot.lds
+++ b/board/eNET/u-boot.lds
@@ -27,7 +27,7 @@ ENTRY(_start)
 
 SECTIONS
 {
-	. = 0x38040000;		/* Location of bootcode in flash */
+	. = 0x06000000;		/* Location of bootcode in flash */
 	_i386boot_text_start = .;
 	.text  : { *(.text); }
 
@@ -97,14 +97,13 @@ SECTIONS
 	 * at reset and the code have to fit.
 	 * The fff0 offset of resetvec is important, however.
 	 */
-
 	. = 0xfffffe00;
-	.start32 : AT (0x3807fe00) { *(.start32); }
+	.start32 : AT (0x0603fe00) { *(.start32); }
 
 	. = 0xf800;
-	.start16 : AT (0x3807f800) { *(.start16); }
+	.start16 : AT (0x0603f800) { *(.start16); }
 
 	. = 0xfff0;
-	.resetvec : AT (0x3807fff0) { *(.resetvec); }
+	.resetvec : AT (0x0603fff0) { *(.resetvec); }
 	_i386boot_end = (LOADADDR(.resetvec) + SIZEOF(.resetvec) );
 }
diff --git a/cpu/i386/start.S b/cpu/i386/start.S
index 1980f1a..e1d4492 100644
--- a/cpu/i386/start.S
+++ b/cpu/i386/start.S
@@ -33,7 +33,32 @@
 .type _start, @function
 .globl _i386boot_start
 _i386boot_start:
+	/*
+	 * This is the fail safe 32-bit bootstrap entry point. The
+	 * following code is not executed from a cold-reset (actually, a
+	 * lot of it is, but from real-mode after cold reset. It is
+	 * repeated here to put the board into a state as close to cold
+	 * reset as necessary)
+	 */
+	cli
+	cld
+
+	/* Turn of cache (this might require a 486-class CPU) */
+	movl	%cr0, %eax
+	orl	$0x60000000,%eax
+	movl	%eax, %cr0
+	wbinvd
+
+	lidt	blank_idt_ptr
+
+	/* Tell 32-bit code it is being entered from  */
+	movw	$0x0000, %bx
+
+	/* Flush the prefetch queue */
+	jmp	_start
 _start:
+	/* This is the 32-bit cold-reset entry point */
+
 	movl    $0x18,%eax	/* Load our segement registes, the
 				 * gdt have already been loaded by start16.S */
 	movw    %ax,%fs
@@ -42,6 +67,15 @@ _start:
 	movw    %ax,%es
 	movw    %ax,%ss
 
+	/*
+	 * Skip low-level board and memory initialization if not starting
+	 * from cold-reset. This allows us to do a fail safe boot-strap
+	 * into a new build of U-Boot from a known-good boot flash
+	 */
+	movw	$0x0001, %ax
+	cmpw	%ax, %bx
+	jne	mem_init_ret
+
 	/* We call a few functions in the board support package
 	 * since we have no stack yet we'll have to use %ebp
 	 * to store the return address */
diff --git a/cpu/i386/start16.S b/cpu/i386/start16.S
index 1ebb6bc..1caa686 100644
--- a/cpu/i386/start16.S
+++ b/cpu/i386/start16.S
@@ -45,7 +45,7 @@ board_init16_ret:
 	wbinvd
 
 	/* load the descriptor tables */
-o32 cs	lidt	idt_ptr
+o32 cs	lidt	blank_idt_ptr
 o32 cs	lgdt	gdt_ptr
 
 
@@ -57,6 +57,8 @@ o32 cs	lgdt	gdt_ptr
 	/* Flush the prefetch queue */
 	jmp	ff
 ff:
+	/* Tell 32-bit code it is being entered from hard-reset */
+	movw	$0x0001, %bx
 
 	/* Finally jump to the 32bit initialization code */
 	movw	$code32start, %ax
@@ -68,7 +70,8 @@ code32start:
 	.long	_start		/* offset */
 	.word	0x10		/* segment */
 
-idt_ptr:
+.global blank_idt_ptr
+blank_idt_ptr:
 	.word	0		/* limit */
 	.long	0		/* base */
 
-- 
1.7.0.2.182.ge007