x86: Support a chained-boot development flow
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>
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 @@
/* 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 @@
.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