[U-Boot] [PATCH v2 12/12] x86: Update REAME.x86 for coreboot support

[Bin Meng]

Update README.x86 to include new build instructions for U-Boot as
the coreboot payload and testing considerations with coreboot.

Signed-off-by: Bin Meng <bmeng.cn at gmail.com>

---

Changes in v2:
- New patch to update REAME.x86 for coreboot support

 doc/README.x86 | 35 +++++++++++++++++++++++++++++++++++
 1 file changed, 35 insertions(+)

diff --git a/doc/README.x86 b/doc/README.x86
index 5fab044..da0c712 100644
--- a/doc/README.x86
+++ b/doc/README.x86
@@ -32,6 +32,21 @@ on other architectures, like below:
 $ make coreboot-x86_defconfig
 $ make all
 
+Note this default configuration will build a U-Boot payload for the Link board.
+To build a coreboot payload aginst another board, you can change the build
+configuration during the 'make menuconfig' process.
+
+x86 architecture  --->
+	...
+	(chromebook_link) Board configuration file
+	(chromebook_link) Board Device Tree Source (dts) file
+	(0x19200000) Board specific Cache-As-RAM (CAR) address
+	(0x4000) Board specific Cache-As-RAM (CAR) size
+
+Change the 'Board configuration file' and 'Board Device Tree Source (dts) file'
+to point to a new board. You can also change the Cache-As-RAM (CAR) related
+settings here if the default values do not fit your new board.
+
 Building ROM version of U-Boot (hereafter referred to as u-boot.rom) is a
 little bit tricky, as generally it requires several binary blobs which are not
 shipped in the U-Boot source tree. Due to this reason, the u-boot.rom build is
@@ -93,6 +108,26 @@ Now you can build U-Boot and obtaim u-boot.rom
 $ make crownbay_defconfig
 $ make all
 
+Test with coreboot
+------------------
+For testing U-Boot as the coreboot payload, there are things that need be paid
+attention to. coreboot supports loading an ELF executable and a 32-bit plain
+binary, as well as other supported payloads. With the default configuration,
+U-Boot is set up to use a separate Device Tree Blob (dtb). As of today, the
+generated u-boot-dtb.in needs to be packaged by the cbfstool utility (a tool
+provided by coreboot) manually as coreboot's 'make menuconfig' does not provide
+this capability yet. The command is as follows:
+
+# in the coreboot root directory
+$ ./build/util/cbfstool/cbfstool build/coreboot.rom add-flat-binary \
+  -f u-boot-dtb.bin -n fallback/payload -c lzma -l 0x1110000 -e 0x1110015
+
+Make sure 0x1110000 matches CONFIG_SYS_TEXT_BASE and 0x1110015 matches the
+symbol address of _start (in arch/x86/cpu/start.S).
+
+If you want to use ELF as the coreboot payload, change U-Boot configuration to
+use CONFIG_OF_EMBED.
+
 CPU Microcode
 -------------
 Modern CPU usually requires a special bit stream called microcode [5] to be
-- 
1.8.2.1