[M] Change in coreboot[main]: nb/sandybridge: Implement adjustable DRAM voltages

[Riku Viitanen (Code Review)]

Attention is currently required from: Angel Pons, Intel coreboot Reviewers, Patrick Rudolph, Riku Viitanen.

Hello Angel Pons, Intel coreboot Reviewers, build bot (Jenkins),

I'd like you to reexamine a change. Please visit

https://review.coreboot.org/c/coreboot/+/85793?usp=email

to look at the new patch set (#12).

The following approvals got outdated and were removed:
Code-Review+1 by Angel Pons, Verified+1 by build bot (Jenkins)

Change subject: nb/sandybridge: Implement adjustable DRAM voltages
......................................................................

nb/sandybridge: Implement adjustable DRAM voltages

Many mainboards implement adjustable voltage rails for DRAM and other
components. For example, on cold start, ASRock Z77 Extreme4's DRAM
voltage regulator defaults to 1.6V and an SMBus connected current DAC
is used to offset this to achieve the desired operating voltage.

Sensible default limits of 1.25V-1.65V are implemented. Mainboards
define their hardware's limits in Kconfig. Users can choose between
the default automatic selection mode and select upper and lower limits
or set a voltage manually in Kconfig, within mainboard hardware limits.

Automatic voltage selection takes into account both SPD and XMP data.
Care is taken to not undervolt or overvolt any modules according to
their own reported specifications. Current choice favours higher
performance. Still, if no XMP profile is used due to their requested
voltages being too high for any other DIMM or the system, or XMP
data simply not existing, the lowest supported voltage is used instead.

This strategy is similar enough to at least two OEM firmwares observed:
ASRock Z77 Extreme4 and ASUS P8H67-I DELUXE. ASRock was tested with
i7-3770K and DDR3 XMP and non-XMP DIMMs in various combinations.
ASUS was tested with i5-2500K and various DDR3 and DDR3L SO-DIMMs.

Since hardware implementations differ, once suitable voltage is
selected, a callback "set_dram_voltage()" is called to do the actual
voltage setting. Mainboards need to implement this in romstage, or
use a driver like CB:85887.

Change-Id: I1a8857deee85fd635429afd3cbf93cad7a7d589b
Signed-off-by: Riku Viitanen 
---
M src/northbridge/intel/sandybridge/Kconfig
M src/northbridge/intel/sandybridge/raminit.c
M src/northbridge/intel/sandybridge/raminit_common.h
M src/northbridge/intel/sandybridge/raminit_native.c
M src/northbridge/intel/sandybridge/sandybridge.h
5 files changed, 130 insertions(+), 5 deletions(-)

git pull ssh://review.coreboot.org:29418/coreboot refs/changes/93/85793/12