Documentation: Add PC Engines apu2

Describe how to run coreboot on the PC Engines apu2 mainboard.

Change-Id: Id1d553c7f7485358960d92e714d50ba0f75b3581
Signed-off-by: Piotr Kleinschmidt <piotr.kleinschmidt@3mdeb.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/33146
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Kyösti Mälkki <kyosti.malkki@gmail.com>

Documentation/mainboard/index.md

@@ -86,6 +86,10 @@ The boards in this section are not real mainboards, but emulators.
 
 - [MS-7707](msi/ms7707/ms7707.md)
 
+## PC Engines
+
+- [APU2](pcengines/apu2.md)
+
 ## Roda
 
 - [RK9 Flash Header](roda/rk9/flash_header.md)

Documentation/mainboard/pcengines/apu2.md

@@ -0,0 +1,116 @@
+# PC Engines APU2
+
+This page describes how to run coreboot on PC Engines APU2 platform.
+
+## Technology
+
+```eval_rst
++------------+---------------------------------------------------------------+
+| CPU        | AMD G series GX-412TC                                         |
++------------+---------------------------------------------------------------+
+| CPU core   | 1 GHz quad Puma core with 64 bit support                      |
+|            | 32K data + 32K instruction cache per core, shared 2MB L2 cache|
++------------+---------------------------------------------------------------+
+| DRAM       | 2 or 4 GB DDR3-1333 DRAM                                      |
++------------+---------------------------------------------------------------+
+| Boot       | From SD card, USB, mSATA SSD, SATA                            |
++------------+---------------------------------------------------------------+
+| Power      | 6 to 12W of 12V power                                         |
++------------+---------------------------------------------------------------+
+| Firmware   | coreboot with support for iPXE and USB boot                   |
++------------+---------------------------------------------------------------+
+```
+
+## Required proprietary blobs
+
+To build working coreboot image some blobs are needed.
+
+```eval_rst
++-----------------+---------------------------------+---------------------+
+| Binary file     | Apply                           | Required / Optional |
++=================+=================================+=====================+
+| amdfw.rom*      | AMD Platform Security Processor | Required            |
++-----------------+---------------------------------+---------------------+
+| AGESA.bin       | AGESA Platform Initialization   | Required            |
++-----------------+---------------------------------+---------------------+
+| xhci.bin        | AMD XHCI controller             | Optional            |
++-----------------+---------------------------------+---------------------+
+```
+(\*) - package containing all required blobs for PSP. Directory, in which all
+blobs are listed and available is: *3rdparty/southbridge/amd/avalon/PSP*
+
+## Flashing coreboot
+
+```eval_rst
++---------------------+--------------------------+
+| Type                | Value                    |
++=====================+==========================+
+| Socketed flash      | no                       |
++---------------------+--------------------------+
+| Model               | W25Q64                   |
++---------------------+--------------------------+
+| Size                | 8 MiB                    |
++---------------------+--------------------------+
+| Package             | SOIC-8                   |
++---------------------+--------------------------+
+| Write protection    | jumper on WP# pin*       |
++---------------------+--------------------------+
+| Dual BIOS feature   | no                       |
++---------------------+--------------------------+
+| Internal flashing   | yes                      |
++---------------------+--------------------------+
+```
+(\*) - It is used in normal SPI mode, but can be dangerous when using Quad SPI
+Flash. Then, pull-down resistors should be considered rather than jumper.
+
+### Internal programming
+
+The SPI flash can be accessed using [flashrom].
+
+    flashrom -p internal -w coreboot.rom
+
+### External programming
+
+**IMPORTANT**: When programming SPI flash, first you need to enter apu2 in S5
+(Soft-off) power state. S5 state can be forced by shorting power button pin on
+J2 header.
+
+The external access to flash chip is available through standard SOP-8 clip or
+SOP-8 header next to the flash chip on the board. Notice that not all boards
+have a header soldered down originally. Hence, there could be an empty slot with
+8 eyelets, so you can solder down a header on your own. The SPI flash chip and
+SPI header are marked in the picture below. Also there is SPI header and SPI
+flash pin layout included. Depend on using header or clip there are important
+rules:
+- using header J6 - don't connect 1,7,8 pins
+- using clip U23 - don't connect 3,7,8 pins
+
+Also signatures at the schematic can be ambiguous:
+- J6 SPIDI = U23 SO = MISO
+- J6 SPIDO = U23 SI = MOSI
+
+There is no restrictions as to the programmer device. It is only recommended to
+flash firmware without supplying power. External programming can be performed,
+for example using OrangePi and Armbian. You can exploit linux_spi driver which
+provides communication with SPI devices. Example command to program SPI flash
+with OrangePi using linux_spi:
+
+    flashrom -f -w coreboot.rom -p linux_spi:dev=/dev/spidev1.0,spispeed=16000
+
+**apu2 platform with marked in SPI header and SPI flash chip**
+
+![][apu2_flash]
+
+**SPI header pin layout**
+
+![][spi_header]
+
+## Schematics
+
+PC Engines APU2 [platform schematics](https://pcengines.ch/schema/apu2d.pdf)
+are available for free on PC Engines official site. Both configurations
+(2GB/4GB) have the same PCB and schematic.
+
+[apu2_flash]: apu2.jpg
+[spi_header]: apu2_spi.jpg
+[flashrom]: https://flashrom.org/Flashrom