On TGL platform, when enable SR_IOV for PCI enumeration, system
hung due to insufficient PCI resource. GFX VF needs lots of MMIO
resource and it cannot be satisfied by SBL in 32 bit mode.
To address this issue, this patch extends the bus 0 downgrade
policy to further allow downgrading PCI bus 0 devices except for
GFX. Now the DowngradeBus0 policy has following values:
0: Do not downgrade PCI devices on bus 0
1: Downgrade all PCI devices on bus 0
2: Downgrade all PCI devices on bus 0 but GFX
3: Reserved
By default, it has the same behavior as before. If platform needs
to download bus 0 devices but GFX, the new value 2 can be used.
This has been tested on TGL, and it worked as expected.
Signed-off-by: Maurice Ma <maurice.ma@intel.com>
c03faf59c3