documentation: Add documentation for Purism Librem 14

Change-Id: Ic68a3d17534f78dae8c432253982e8d10a6427f0
Signed-off-by: Matt DeVillier <matt.devillier@puri.sm>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/51550
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>

Documentation/mainboard/purism/librem_14.md

@@ -0,0 +1,117 @@
+# Purism Librem 14
+
+This page describes how to run coreboot on the [Purism Librem 14].
+
+```eval_rst
++------------------+------------------------------------------------------+
+| CPU              | Intel Core i7-10710U                                 |
++------------------+------------------------------------------------------+
+| PCH              | Comet Lake LP Premium (Comet Lake-U)                 |
++------------------+------------------------------------------------------+
+| EC               | ITE IT8528E                                          |
++------------------+------------------------------------------------------+
+| Coprocessor      | Intel Management Engine (CSME 14.x)                  |
++------------------+------------------------------------------------------+
+```
+
+![](librem_14.webp)
+![](librem_14_flash.jpg)
+![](librem_14_ec_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            |
++-----------------+---------------------------------+---------------------+
+```
+
+FSP-M and FSP-S are obtained after splitting the CometLake1 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.
+
+A VGA Option ROM is not required to boot, as the Librem 14 uses libgfxinit.
+
+## Intel Management Engine
+
+The Librem 14 uses version 14.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 checking the coreboot console log, using coreboot’s
+cbmem utility:
+
+    `sudo ./cbmem -1 | grep 'ME:'`
+
+provided coreboot has been patched 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]. No official flashrom
+release supports the CometLake-U SoC yet, so it must be built from source.
+Version v1.2-107-gb1f858f or later is needed. 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 16 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, in between the CPU
+heatsink and the left cooling fan, just above the left SO-DIMM slot.
+
+One has to remove all 9 screws from the bottom cover, then disconnect the
+battery from the mainboard (bottom left of mainboard). Use a SOIC-8 chip
+clip to program the chip (a Gigadevice GD25Q127C (3.3V) - [datasheet][GD25Q127C]).
+
+The EC firmware is stored on a separate SOIC-8 chip (a Gigadevices GD25Q80C),
+located underneath the Wi-Fi module, below the left cooling fan.
+
+## Known issues
+
+ * Automatic detection of external audio input/output via the 3.5mm jack
+   does not currently work.
+ * Touchpad interrupt non-functional when correctly configured as active-low
+   and level-triggered, so configured as active-high instead. This causes an
+   interrupt storm and prevents the CPU from idling due to moderate (10-20%)
+   usage on a single core.
+ * PL1/PL2 limited to 15W/20W by charger and battery discharge capability,
+   not SoC or thermal design.
+
+## Working
+
+ * Internal display with libgfxinit, VGA option ROM, or FSP/GOP init
+ * External displays via HDMI, USB-C Alt-Mode
+ * SeaBIOS (1.14), Tianocore (CorebootPayloadPkg), and Heads payloads
+ * Ethernet, m.2 2230 Wi-Fi
+ * System firmware updates via flashrom
+ * M.2 storage (NVMe, SATA III)
+ * Built-in audio (speakers, microphone)
+ * SMBus (reading SPD from DIMMs)
+ * Initialization with FSP 2.0 (CometLake1)
+ * S3 Suspend/Resume
+ * Booting PureOS 10.x, Debian 11.x, Qubes 4.0.4, Windows 10 20H2
+
+## Not working / untested
+
+ * N/A
+
+
+[Purism Librem 14]: https://puri.sm/products/librem-14/
+[purism-blobs]: https://source.puri.sm/coreboot/purism-blobs
+[GD25Q127C]: https://www.gigadevice.com/datasheet/gd25q127c/
+[flashrom]: https://flashrom.org/Flashrom