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

[Bin Meng]

Hi Simon,

On Sat, Mar 12, 2016 at 1:04 PM, Simon Glass <sjg at chromium.org> wrote:
> 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.

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 <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.

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