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 ---
Changes in v3: - Add more detail to the commit message and code comments
Changes in v2: None
arch/x86/cpu/start.S | 80 ++++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 80 insertions(+)
diff --git a/arch/x86/cpu/start.S b/arch/x86/cpu/start.S index 485868f..a5cba1c 100644 --- a/arch/x86/cpu/start.S +++ b/arch/x86/cpu/start.S @@ -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