[coreboot] Hint for boot from a PCI device.

[vvv at ru.ru]

How about debuging/loading/using coreboot from a PCI 
device?
Quote from Intel I/O Controller Hub 9 (ICH9) Family 
Datasheet:

CS—General Control and Status Register
Offset Address: 3410–3413h              Attribute: R/W, 
R/WLO
Default Value:  00000yy0h (yy = xx0000x0b)Size:    32-bit

Boot BIOS Straps (BBS) — R/W. This field determines the 
destination of accesses
to the BIOS memory range. The default values for these 
bits represent the strap
values of GNT0# (bit 11) at the rising edge of PWROK and 
SPI_CS1#/GPIO58
(Desktop Only) /CLGPIO6 (Digital Office Only) (bit 10) at 
the rising edge of
CLPWROK.

         Bits 11:10     Description
             0xb        SPI
             10b        PCI
             11b        LPC
When PCI is selected, the top 16MB of memory below 4GB 
(FF00_0000h to 11:10 FFFF_FFFFh)
is accepted by the primary side of the PCI P2P bridge and 
forwarded to
the PCI bus. This allows systems with corrupted or 
unprogrammed flash to boot from
a PCI device. The PCI-to-PCI bridge Memory Space Enable 
bit does not need to be set
(nor any other bits) in order for these cycles to go to 
PCI. Note that BIOS decode
range bits and the other BIOS protection bits have no 
effect when PCI is selected.
This functionality is intended for debug/testing only.

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