Hi Bin,
On 11 March 2016 at 01:46, Bin Meng bmeng.cn@gmail.com wrote:
Hi Simon,
On Mon, Mar 7, 2016 at 10:28 AM, Simon Glass sjg@chromium.org wrote:
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.
I don't understand, why is coreboot having a problem here?
It's not coreboot, it's that 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. This is just a hack so you can jump to U-Boot from any stage of coreboot, not just at the end.
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.
Signed-off-by: Simon Glass sjg@chromium.org
arch/x86/cpu/start.S | 79 ++++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 79 insertions(+)
diff --git a/arch/x86/cpu/start.S b/arch/x86/cpu/start.S index 485868f..5fbc2b5 100644 --- a/arch/x86/cpu/start.S +++ b/arch/x86/cpu/start.S @@ -18,6 +18,13 @@ #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 coreboot, for example.
- This is only useful for development.
- */
+#undef LOAD_FROM_32_BIT
.section .text .code32 .globl _start @@ -68,6 +75,10 @@ _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 +231,71 @@ 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
Regards, Bin
Regards, Simon