[coreboot] Boots from PCI add-on card on Intel ICHs

[Kyösti Mälkki]

On Sun, 2012-05-20 at 11:23 +0300, Kyösti Mälkki wrote:
> Hi
> 
> I did not find this method of bypassing the mainboard flash chip and
> booting from PCI add-on card documented or discussed before. The nice
> think in this is that neither mainboard or its flash needs to be
> modified. Good news in the case of a soldered flash and this method may
> work with mini-PCI slots on laptops too.
> 
> For pre-ICH6 the key is in subtractive PCI decode. This has been
> supported in 82801 chipset from the early days and is briefly documented
> in ICH3 datasheet [1], see 5.1.1. PCI Bus interface. This decode mode is
> on by default and there is no documentation of a hw bootstrap that could
> disable it.

Above is not accurate, ICH6 has hardware bootstrap.

> For ICH7 onwards there are HW bootstraps to select between LPC/SPI/PCI.
> If you don't know where the bootstraps are, go with SPI and forget about
> this PCI add-on boot.

The bootstrap is latched on power cycle, but not PCI reset, and the
config bit is writeable. So if one has a booting system it should be
possible to switch to PCI add-on for next reboot. The setting will
default back to mainboard flash after power-cycle. 

Note that "hard/cold" reboot is required to toggle the PCI RST#. If
vendor BIOS does set the write-protection lock bit, it should be cleared
on PCI RST#. At least for ICH4 that is the case.

Kyösti