Hi Simon, On Sat, Mar 12, 2016 at 1:04 PM, Simon Glass <s...@chromium.org> wrote: > Hi Bin, > > On 11 March 2016 at 01:46, Bin Meng <bmeng...@gmail.com> wrote: >> Hi Simon, >> >> On Mon, Mar 7, 2016 at 10:28 AM, Simon Glass <s...@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.
OK, please add some clarification in the commit message and/or the code comments below. > >> >>> >>> 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 <s...@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. Needs some clarifications on "boot directly from coreboot" >>> + * 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 _______________________________________________ U-Boot mailing list U-Boot@lists.denx.de http://lists.denx.de/mailman/listinfo/u-boot