[BUG] i2c_piix4: Hudson2 uses wrong port to access SMBus controller

[Will Wagner <willw@xxxxxxxxxxxx>]

Hello,

I have a Fujitsu DS3313-S6 mainboard with a GX-424CC apu on it 
(http://www.fujitsu.com/fts/products/computing/peripheral/mainboards/industrial-mainboards/d3313s.html). 
It identifies as having PCI device 0x1022:780b which is 
PCI_DEVICE_ID_AMD_HUDSON2_SMBUS

The i2c-piix4.c driver was broken for me by commit 6befa3fde65f i2c: 
piix4: Support alternative port selection register

In that commit it added support for accessing the SMBus from either 0x2c 
or alternatively 0x2e. The commit log comment says that the alternative 
register is the only method supported for the Hudson2 chipset and so in 
the code if the PCIe vendorId is AMD it is hard coded to use the 
alternative register.

Use of this alternative register on my board produces random locks and 
i2c transaction errors. If I instead change the code so that it reads 
Smbus0SelEn (0x2f.) to decide how to access it then it decides on 
Smbus0En (0x2c) and everything works again great.

There is no justification in the commit message for why only the 
alternative is supported for Hudson2. I have tried looking in the AMD 
documentation, unfortunately the BKDG for family 16h models 30h-3Fh 
doesn't seem to document any of these registers (whereas it is covered 
in the SB800 reference guide).

I'd love to submit a patch that fixes this issue, but without better 
documentation I'm not sure if it is invalid on some hardware to check 
Smbus0SelEn to decide on the port. Does anyone have any idea of exactly 
what the correct behaviour should be?

Thanks
Will