[U-Boot] [PATCH 62/69] x86: Support a chained-boot development flow

[Bin Meng]

Hi Simon,

On Mon, Mar 7, 2016 at 10:28 AM, Simon Glass <sjg at 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?

>
> 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 at 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