Documentation: Add basic flashing tutorial for Lenovo

* Add basic flashing tutorial
** Describe internal and external flashing
** Describe flash supply diode protection
** Gives general advices on flashing
** Describe how to use flashrom --ifd
*  Describe basic flashing on Lenovo T4xx devices
** Describe how to disassemble and access the flash IC on T4xx
** Describe flash layout on Sandy Bridge and Ivy Bridge series.

Change-Id: Ia833e27f4e7d89ee32be9bed21a0c021839facec
Signed-off-by: Patrick Rudolph <siro@das-labor.org>
Reviewed-on: https://review.coreboot.org/27852
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Philipp Deppenwiese <zaolin.daisuki@gmail.com>

Documentation/flash_tutorial/index.md

@@ -0,0 +1,105 @@
+# Flashing firmware tutorial
+
+Updating the firmware is possible using the **internal method**, where the updates
+happen from a running system, or using the **external method**, where the system
+is in a shut down state and an external programmer is attached to write into the
+flash IC.
+
+## Contents
+
+* [Flashing internaly](int_flashrom.md)
+* [Flashing firmware standalone](ext_standalone.md)
+* [Flashing firmware externally supplying direct power](ext_power.md)
+* [Flashing firmware externally without supplying direct power](no_ext_power.md)
+
+## General advice
+
+* It's recommended to only flash the BIOS region.
+* Always verify the firmware image.
+* If you flash externally and have transmission errors:
+  * Use short wires
+  * Reduce clock frequency
+  * Check power supply
+  * Make sure that there are no other bus masters (EC, ME, SoC, ...)
+
+## Internal method
+
+This method using [flashrom] is available on many platforms, as long as they
+aren't locked down.
+
+There are various protection schemes that make it impossible to modify or
+replace a firmware from a running system. coreboot allows to disable these
+mechanisms, making it possible to overwrite (or update) the firmware from a
+running system.
+
+Usually you must use the **external method** once to install a retrofitted
+coreboot and then you can use the **internal method** for future updates.
+
+There are multiple ways to update the firmware:
+* Using flashrom's *internal* programmer to directly write into the firmware
+  flash IC, running on the target machine itself
+* A proprietary software to update the firmware, running on the target machine
+  itself
+* A UEFI firmware update capsule
+
+More details on flashrom's
+* [internal programmer](int_flashrom.md)
+
+## External method
+
+External flashing is possible on many platforms, but requires disassembling
+the target hardware. You need to buy a flash programmer, that
+exposes the same interface as your flash IC (likely SPI).
+
+Please also have a look at the mainboard-specific documentation for details.
+
+After exposing the firmware flash IC, read the schematics and use one of the
+possible methods:
+
+* [Flashing firmware standalone](ext_standalone.md)
+* [Flashing firmware externally supplying direct power](ext_power.md)
+* [Flashing firmware externally without supplying direct power](no_ext_power.md)
+
+**WARNING:** Using the wrong method or accidentally using the wrong pinout might
+  permanently damage your hardware!
+
+## Using a layout file
+On platforms where the flash IC is shared with other components you might want
+to write only a part of the flash IC. On Intel for example there are IFD, ME and
+GBE which don't need to be updated to install coreboot.
+To make [flashrom] only write the *bios* region, leaving Intel ME and Intel IFD
+untouched, you can use a layout file, which can be created using ifdtool
+
+```bash
+ifdtool -f rom.layout coreboot.rom
+```
+
+and looks similar to:
+
+```
+00000000:00000fff fd
+00500000:00bfffff bios
+00003000:004fffff me
+00001000:00002fff gbe
+```
+
+By specifying *-l* and *-i* [flashrom] writes a single region:
+```bash
+flashrom -l rom.layout -i bios -w coreboot.rom -p <programmer>
+```
+
+## Using an IFD to determine the layout
+flashrom version 1.0 supports reading the layout from the IFD (first 4KiB of
+the ROM). You don't need to manually specify a layout it, but it only works
+under the following conditions:
+
+* Only available on Intel ICH7+
+* There's only one flash IC when flashing externally
+
+```bash
+flashrom --ifd -i bios -w coreboot.rom -p <programmer>
+```
+
+**TODO** explain FMAP regions, normal/fallback mechanism, flash lock mechanisms
+
+[flashrom]: https://www.flashrom.org/Flashrom