Documentation/nb/intel: Add Haswell documentation

At the moment, this just gives some details on the MRC.

Change-Id: I84e8ca2543b2e19b84a24f7d7032a4aedb6e9272
Signed-off-by: Tristan Corrick <tristan@corrick.kiwi>
Reviewed-on: https://review.coreboot.org/c/30356
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Nico Huber <nico.h@gmx.de>

2 files changed, 43 insertions, 0 deletions

diff --git a/Documentation/northbridge/intel/haswell/index.md b/Documentation/northbridge/intel/haswell/index.md
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+# Intel Haswell documentation
+
+This section describes the Intel Haswell architecture as it relates to
+coreboot.
+
+## Proprietary blobs
+
+- [mrc.bin](mrc.bin.md)
diff --git a/Documentation/northbridge/intel/haswell/mrc.bin.md b/Documentation/northbridge/intel/haswell/mrc.bin.md
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+# mrc.bin
+
+All Haswell boards supported by coreboot currently require a proprietary
+blob in order to initialise the DRAM and a few other components. The
+blob, named `mrc.bin`, largely consists of Intel's memory reference code
+(MRC), but it has been tailored specifically for Chrome OS. It is just
+under 200 KiB in size. Another name for `mrc.bin` is the system agent
+binary.
+
+Having a replacement for `mrc.bin` using native coreboot code is very
+much desired, but it is not an easy task.
+
+## Obtaining mrc.bin
+
+Unfortunately, it is not currently possible to distribute `mrc.bin` as
+part of coreboot. Though, it can be obtained from a Haswell Chromebook
+or Chromebox firmware image, and you might find one online. `mrc.bin`
+can be extracted with the following command. If extracting from a
+"standard" coreboot image, omit `-r RO_SECTION`.
+
+```bash
+cbfstool coreboot.rom extract -f mrc.bin -n mrc.bin -r RO_SECTION
+```
+
+Now, place `mrc.bin` in the root of the coreboot directory.
+Alternatively, place `mrc.bin` anywhere you want, and set `MRC_FILE` to
+its location when building coreboot.
+
+## ECC DRAM
+
+When `mrc.bin` has finished executing, ECC is active on the channels
+populated with ECC DIMMs. However, `mrc.bin` was tailored specifically
+for Haswell Chromebooks and Chomeboxes, none of which support ECC DRAM.
+While ECC likely functions correctly, it is advised to further validate
+the correct operation of ECC if data integrity is absolutely critical.