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

[Simon Glass]

Hi Bin,

On 11 March 2016 at 01:46, Bin Meng <bmeng.cn at gmail.com> wrote:
> 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?

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 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

Regards,
Simon