documentation: Add documentation for Purism Librem Mini

Change-Id: Ie5699942f48d2d5b1417f447a9a36b98e4b18156
Signed-off-by: Matt DeVillier <matt.devillier@puri.sm>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/42882
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

Documentation/mainboard/index.md

@@ -136,6 +136,10 @@ The boards in this section are not real mainboards, but emulators.
 
 - [Hermes](prodrive/hermes.md)
 
+## Purism
+
+- [Librem Mini](purism/librem_mini.md)
+
 ## Protectli
 
 - [FW2B / FW4B](protectli/fw2b_fw4b.md)

Documentation/mainboard/purism/librem_mini.md

@@ -0,0 +1,120 @@
+# Purism Librem Mini
+
+```eval_rst
++------------------+--------------------------------------------------+
+| CPU              | Intel Core i7-8665U                              |
++------------------+--------------------------------------------------+
+| PCH              | Whiskey Lake / Cannon Point LP                   |
++------------------+--------------------------------------------------+
+| Super I/O, EC    | ITE IT8528E                                      |
++------------------+--------------------------------------------------+
+| Coprocessor      | Intel Management Engine (CSME 12.x)              |
++------------------+--------------------------------------------------+
+```
+
+![](librem_mini.jpg)
+![](librem_mini_flash.jpg)
+
+## Required proprietary blobs
+
+To build a minimal working coreboot image some blobs are required (assuming
+only the BIOS region is being modified).
+
+```eval_rst
++-----------------+---------------------------------+---------------------+
+| Binary file     | Apply                           | Required / Optional |
++=================+=================================+=====================+
+| FSP-M, FSP-S    | Intel Firmware Support Package  | Required            |
++-----------------+---------------------------------+---------------------+
+| microcode       | CPU microcode                   | Required            |
++-----------------+---------------------------------+---------------------+
+| vgabios         | VGA Option ROM                  | Optional            |
++-----------------+---------------------------------+---------------------+
+```
+
+FSP-M and FSP-S are obtained after splitting the Coffee Lake FSP binary (done
+automatically by the coreboot build system and included into the image) from
+the `3rdparty/fsp` submodule.
+
+Microcode updates are automatically included into the coreboot image by the build
+system from the `3rdparty/intel-microcode` submodule. Official Purism release
+images may include newer microcode, which is instead pulled from Purism's
+[purism-blobs] repository.
+
+VGA Option ROM is not required to boot, but if one needs graphics in pre-OS
+stage, it should be included (if not using FSP/GOP display init). It can
+be extracted via cbfstool from the existing board firmware or pulled from
+the [purism-blobs] repository.
+
+## Intel Management Engine
+
+The Librem Mini uses version 12.x of the Intel Management Engine (ME) /
+Converged Security Engine (CSE). The ME/CSE is disabled using the High
+Assurance Platform (HAP) bit, which puts the ME into a disabled state
+after platform bring-up (BUP) and disables all PCI/HECI interfaces.
+This can be verified via the coreboot cbmem utility:
+`sudo ./cbmem -1 | grep 'ME:'`
+provided coreboot has been modified to output the ME status even when
+the PCI device is not visible/active (as it is in Purism's release builds).
+
+## Flashing coreboot
+
+### Internal programming
+
+The main SPI flash can be accessed using [flashrom]. The first version
+supporting the chipset is flashrom v1.2. Firmware an be easily flashed
+with internal programmer (either BIOS region or full image).
+
+### External programming
+
+The system has an internal flash chip which is a 8 MiB soldered SOIC-8 chip,
+and has a diode attached to the VCC line for in-system programming.
+This chip is located on the bottom side of the board under the CPU heatsink,
+in line with the front USB 2.0 ports.
+
+One has to remove all screws (in order):
+
+ * 2 top cover screws
+ * 4 screws securing the mainboard to the chassis
+ * 4 screws securing the heatsink/fan assembly to the mainboard (under the SODIMMs)
+
+The m.2 SSD will need to be removed if the Wi-Fi antenna are connected to
+an internal Wi-Fi/BT module. Use a SOIC-8 chip clip to program the chip.
+Specifically, it's a Winbond W25Q128JV (3.3V) -[datasheet][W25Q128JV].
+
+The EC firmware is stored on a separate SOIC-8 chip (a Winbond W25Q80DV),
+but is not protected by a diode and therefore cannot be read/written to without
+desoldering it from the mainboard.
+
+## Known issues
+
+ * SeaBIOS can be finicky with detecting USB devices
+ * Booting can sometimes hang when a bootsplash image is used with SeaBIOS
+   and VGA option ROM display init, related to display mode changing
+ * Issues with some SATA devices have been mitigated by limiting the SATA speed to 3Gbps
+   until the correct HSIO PHY settings can be determined.
+
+## Working
+
+ * External displays via HDMI/DislpayPort with VGA option ROM or FSP/GOP init
+  (no libgfxinit support yet)
+ * SeaBIOS (1.13.x), Tianocore (CorebootPayloadpkg), Heads (Purism downstream) payloads
+ * Ethernet, m.2 2230 Wi-Fi
+ * System firmware updates via flashrom
+ * PCIe NVMe
+ * m.2 and SATA III
+ * Audio via front 3.5mm jack, HDMI, and DisplayPort
+ * SMBus (reading SPD from DIMMs)
+ * Initialization with CFL FSP 2.0
+ * S3 Suspend/Resume
+ * Booting PureOS 9.x, Debian 10.x, Qubes 4.0.3, Linux Mint 19.3, Windows 10 2004
+
+## Not working / untested
+
+ * ITE IT8528E Super IO functions
+
+
+[Purism Librem Mini]: https://puri.sm/products/librem-mini/
+[purism-blobs] : https://source.puri.sm/coreboot/purism-blobs
+[W25Q128JV]: https://www.winbond.com/resource-files/w25q128jv%20revf%2003272018%20plus.pdf
+[flashrom]: https://flashrom.org/Flashrom