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x86: Support a chained-boot development flow

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Sometimes it is useful to jump into U-Boot directly from coreboot or UEFI
without any 16-bit init. This can help during development by allowing U-Boot
to avoid doing all the init required by the platform.

U-Boot expects its GDT to be set up correctly by its 16-bit code. If
coreboot doesn't do this (because it hasn't run the payload setup code yet)
then this won't happen.

In this case we cannot rely on the GDT settings. U-Boot will hang or crash
if these are wrong. Provide a development-only option to set up the GDT
correctly. This is just a hack so you can jump to U-Boot from any stage of
coreboot, not just at the end.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
This commit is contained in:
Simon Glass 2016-03-16 07:44:40 -06:00 committed by Bin Meng
parent 69ca6fd850
commit e5aa8a9b15
1 changed files with 80 additions and 0 deletions
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80
arch/x86/cpu/start.S
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@ -18,6 +18,14 @@
#include <generated/generic-asm-offsets.h>
#include <generated/asm-offsets.h>
/*
* Define this to boot U-Boot from a 32-bit program which sets the GDT
* differently. This can be used to boot directly from any stage of coreboot,
* for example, bypassing the normal payload-loading feature.
* This is only useful for development.
*/
#undef LOAD_FROM_32_BIT
.section .text
.code32
.globl _start
@ -68,6 +76,10 @@ _start:
/* Save table pointer */
movl %ecx, %esi
#ifdef LOAD_FROM_32_BIT
lgdt gdt_ptr2
#endif
/* Load the segement registers to match the GDT loaded in start16.S */
movl $(X86_GDT_ENTRY_32BIT_DS * X86_GDT_ENTRY_SIZE), %eax
movw %ax, %fs
@ -220,3 +232,71 @@ multiboot_header:
.long 0
/* entry addr */
.long CONFIG_SYS_TEXT_BASE
#ifdef LOAD_FROM_32_BIT
/*
* The following Global Descriptor Table is just enough to get us into
* 'Flat Protected Mode' - It will be discarded as soon as the final
* GDT is setup in a safe location in RAM
*/
gdt_ptr2:
.word 0x1f /* limit (31 bytes = 4 GDT entries - 1) */
.long gdt_rom2 /* base */
/* Some CPUs are picky about GDT alignment... */
.align 16
.globl gdt_rom2
gdt_rom2:
/*
* The GDT table ...
*
* Selector Type
* 0x00 NULL
* 0x08 Unused
* 0x10 32bit code
* 0x18 32bit data/stack
*/
/* The NULL Desciptor - Mandatory */
.word 0x0000 /* limit_low */
.word 0x0000 /* base_low */
.byte 0x00 /* base_middle */
.byte 0x00 /* access */
.byte 0x00 /* flags + limit_high */
.byte 0x00 /* base_high */
/* Unused Desciptor - (matches Linux) */
.word 0x0000 /* limit_low */
.word 0x0000 /* base_low */
.byte 0x00 /* base_middle */
.byte 0x00 /* access */
.byte 0x00 /* flags + limit_high */
.byte 0x00 /* base_high */
/*
* The Code Segment Descriptor:
* - Base = 0x00000000
* - Size = 4GB
* - Access = Present, Ring 0, Exec (Code), Readable
* - Flags = 4kB Granularity, 32-bit
*/
.word 0xffff /* limit_low */
.word 0x0000 /* base_low */
.byte 0x00 /* base_middle */
.byte 0x9b /* access */
.byte 0xcf /* flags + limit_high */
.byte 0x00 /* base_high */
/*
* The Data Segment Descriptor:
* - Base = 0x00000000
* - Size = 4GB
* - Access = Present, Ring 0, Non-Exec (Data), Writable
* - Flags = 4kB Granularity, 32-bit
*/
.word 0xffff /* limit_low */
.word 0x0000 /* base_low */
.byte 0x00 /* base_middle */
.byte 0x93 /* access */
.byte 0xcf /* flags + limit_high */
.byte 0x00 /* base_high */
#endif