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 485868ff5769ecca47fa01e0028eca14ac4ccdeb..a5cba1cf2a766680e3e1b8ab6290bee2aa8a0191 100644 (file)
--- 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