Re: Two devicetree questions and two use cases

Hi Keith,

I can't speak to the devicetree stuff, but I did find something 
interesting regarding the PCH PCIe ports:

> p8z77-v and sabertooth_z77 have a third PCIe x4 slot serviced by the
> PCH, but the 4 lanes are shared with some other x1 slots and devices.
> They can configure the PCIe x4 slot to really be x4, or the 4 lanes
> can be configured as 4 x1 ports for various x1 slots or onboard
> devices. This requires changing a soft strap as well as manipulating
> some GPIOs in the SIO chip, and the devicetree changed to match. I
> have the SIO GPIO part figured out, the soft strap I can only come to
> one possible explanation: reflash SPI descriptor with the changed soft
> strap and force a platform reset. To verify I'll need to get a board
> and actually check the flash chip for changes.

Changing the IFD at runtime seems like an impractical way to achieve 
this, especially since the IFD is normally supposed to be read only. I 
suspect there's another, undocumented method to change the 
configuration. On ICH/PCH datasheets, there is an RCBA register (Root 
Port Configuration Register) that indicates the current port 
configuration. On the 7 series PCH's, that is at RCBA32(0x400), bits 3:2 
for ports 5-8 and bits 1:0 for ports 1:4. However, these bitfields are 
documented as read only in the public datasheets. However, in the public 
ICH9 datasheet (and seemingly only the ICH9 datasheet; other datasheets 
like ICH7, ICH8, ICH10, etc don't have this detail) the equivalent 
register at RCBA32(0x224) lists the bitfields as R/W, with a note saying 
that writing to them is for debug/testing and should be treated as read 
only and modifiable only through hard straps (it seems like ICH9 didn't 
use IFD soft straps for this). Based on that note, I assume this 
mechanism is intended for testing different PCH PCIe configurations 
without reflashing the IFD. I haven't tested this yet on any platform 
but it does seem plausible that this mechanism exists on chipsets other 
than ICH9.

Cheers,
Nicholas
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