Hi Bin, On 11 March 2016 at 01:46, Bin Meng <[email protected]> wrote: > Hi Simon, > > On Mon, Mar 7, 2016 at 10:28 AM, Simon Glass <[email protected]> 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 <[email protected]> >> --- >> >> 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 _______________________________________________ U-Boot mailing list [email protected] http://lists.denx.de/mailman/listinfo/u-boot
