Documentation/flashing_firmware: Add info about SPI flash header

Some mainboards have a header connected to the SPI bus, which can be
used to connect a second flash chip and override the onboard flash. This
allows one to boot coreboot on the system without ever having to flash
the onboard flash. HP boards with this header all seem to use the same
2x8 or 2x10 header layout, so document the pinout.

Change-Id: Ic2bf1244adfb78872340f212519c6ab33e26646a
Signed-off-by: Nicholas Chin <nic.c3.14@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/67818
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

Documentation/tutorial/flashing_firmware/ext_standalone.md

@@ -19,5 +19,45 @@ It is possible to set the onboard flash on hold and use another flash chip.
 Connect all lines one-to-one, except /HOLD. Pull /HOLD of the soldered flash IC
 low, and /HOLD of your replacement flash IC high.
 
+### SPI header
+Some boards feature a pin header which is connected to the SPI bus. This can
+also be used to connect a secondary flash chip.
+
+#### HP boards
+Many HP desktop mainboards have a 2x8 or 2x10 block of header pins which
+can be used to connect a second flash chip. One pin is connected to the
+onboard flash's /CS pin, and another is connected to the chipset's /CS
+pin. Normally a jumper cap is placed between these pins, allowing the
+chipset to access the onboard flash. To use this header, remove this
+jumper and connect all lines except /CS one-to-one with the
+corresponding pin on the header. The secondary flash chip's /CS line
+should be connected to the chipset /CS pin on the header. By doing this
+the secondary SPI flash, rather than the onboard flash, will respond to
+accesses from the chipset.
+
+![][hp_spi_header_pinout]
+
+![][hp_spi_header_mainboard]
+
+Note that on boards where this header is unpopulated, a jumper resistor
+will be populated nearby which serves the purpose of the jumper cap. One
+end should have continuity with the /CS pin on the flash, and the other
+end should have continuity with the chipset /CS pin on the unpopulated
+header. It may also be possible to find this resistor through visual
+inspection of the traces on the /CS pins. This resistor should be
+desoldered if you wish to solder a pin header to the board and connect a
+secondary flash, otherwise the onboard flash will always respond to
+accesses.
+
+#### Other boards
+Other boards may have similar headers, but will likely use a different
+pinout than the ones explicitly mentioned here. Usually such a header
+will be located near the onboard SPI flash. If schematics are available,
+the pinout for the header will likely be found in it, but it could also
+be determined using a multimeter in continuity mode to map out the
+connections between the SPI flash and the header.
+
 
 [EM100Pro]: https://www.dediprog.com/product/EM100Pro
+[hp_spi_header_pinout]: hp_spi_header.svg
+[hp_spi_header_mainboard]: hp_spi_header_mb.jpg

Documentation/tutorial/flashing_firmware/hp_spi_header.svg

@@ -0,0 +1,109 @@
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