[coreboot] Understand vboot logic to select FW_B

[Aaron Durbin]

On Tue, Feb 14, 2017 at 8:26 PM,  <tturne at codeaurora.org> wrote:
>
> Folks,
> I'm working on coreboot on ARMv8 architecture and recently attempted to
> verify FW_B can be selected if FW_A is corrupted.
>
> In reviewing the code-path through coreboot vboot wrapper and
> vboot_reference library I'm not understanding the logic.
>
> The logic appears to be that vboot_reference library determines FW_A is
> corrupt and forces a reboot.
>
> After reboot, early in verstage_main(), both the NVRAM (flash) and
> vb2_context structures are initialized, wiping out any history of failure
> condition.
>
> I know I must be missing something in my analysis but don't see it.
>
> Can anybody on this list share their experiences in confirming the FW_B boot
> path if FW_A is corrupt?


ctx->flags & VB2_CONTEXT_FW_SLOT_B determines which slot to use. The
vbnv (vboot non-volatile) storage keeps counts for slots to try.
vb2_select_fw_slot() will do that work for you. It's located in
firmware/2lib/2misc.c. That function is called from vb2api_fw_phase2()
in firmware/2lib/2api.c.  All this logic lives in vboot_reference code
repository.


> Cheers,
> T.mike
>
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