Add documentation for monitor TUI application

- add documentation for pkg/monitor container
- update NIM documentation
- add documentation for pkg/pillar/cmd/monitor service

Signed-off-by: Mikhail Malyshev <mike.malyshev@gmail.com>

docs/LOCAL-TUI.md

@@ -0,0 +1,41 @@
+# TUI (Text User Interface) for the local operator
+
+EVE has a user-friendly TUI (Text User Interface) that can be used to interact with the system.
+The implementation is consists of two parts
+
+1. Client application responsible for rendering the TUI, sending user input to the server, and handling asynchronous server notification. The client is written in Rust and hosted at [https://github.com/lf-edge/eve-monitor-rs](https://github.com/lf-edge/eve-monitor-rs). Corresponding Dockerfile and LinuxKit build files are located at `pkg/monitor`
+2. Server part is implemented inside [pkg/pillar/cmd/monitor](../pkg/pillar/cmd/monitor/)
+
+The client communicates with the server over UNIX socket
+
+## TTY and serial console
+
+The UI is rendered on a local TTY (/dev/tty2) only i.e. on a physical monitor attached to the system. Neither Serial Console nor SSH connection has access to TUI. It is done to ensure the physical presence of the operator.
+
+## /dev/ttyX vs /dev/console
+
+There are two distinguishable console devices in Linux `/dev/console` and `/dev/ttyX`. The later points to a particular virtual terminal device and the former points to *currently active* TTY device. The user can switch between virtual terminals by using `Alt+Fx` or `Alt+<,>` keys. When the current TTY is set `/dev/console` tracks this change and always points to to the current terminal
+
+Monitor application is spawned on a `/dev/tty2` using a `openvt` utility while the rest of the applications are spawned on the default kernel console defined by `console=` parameters on the kernel command line. When the application is in focus (`/dev/tty2` is an active console) writing to `/dev/console` which points to the same device corrupts TUI thus it cannot be used by other services in the system to produce the output. At least when `/dev/tty2` is a current console.
+
+From the other hand `/dev/tty`  (no digit at the end!) device always points to a TTY *in the context of running process*. This device can be used instead of `/dev/console` by other services for the output.
+
+## Limitations of linux terminal
+
+Rust application can be built and run on Linux host for testing and development purposes. When running on a host its terminal is used for rendering. Host terminals ( e.g. `TERM=xterm`) are very different in capabilities compared to the built in Linux terminal which is used for `/dev/ttyX` (`TERM=linux`) devices. The major differences important for monitor application are
+
+* Number of supported colors.
+
+  `linux` terminal can use only 8 colors for foreground and 8 colors for background colors. In contrast host terminals can easily display 256 colors and more
+
+* Limited number of pseudo-graphics glyphs.
+
+  These limitations can be relaxed by using a custom font with 256 glyphs compared to the standard one that uses 512 glyphs. In this case an extra bit can be used to render 16 colors. Besides, extra pseudo-graphics glyphs can be added instead of unused characters to display e.g. rounded boxes.
+
+  As of now a standard font is used so the look of the application on the host and on EVE is different
+
+* Key handling.
+
+  By default `linux` terminal cannot properly handle many key combinations e.g. `PgDwn`, `Ctrl+left, Ctrl + right`, etc. A custom key map must be set to properly handle required combinations. It is done in [pkg/monitor/run-monitor.sh](../pkg/monitor/run-monitor.sh) by calling `loadkeys` utility
+
+  As of now only `Ctrl + [left|right|up|down]` are properly handled.

pkg/pillar/docs/monitor.md

@@ -0,0 +1,17 @@
+# Monitor service implementation
+
+The monitor service is a simple IPC server which uses a unix socket to communicate with external [rust client](../../monitor/Dockerfile) located at `pkg/monitor`. The server can send asynchronous updates about EVE status to the
+connected client. The information is then used by rust client to display a TUI for the user.
+
+## Client requests
+
+Following requests are supported at the moment:
+
+* `SetDPC` - sets a the current DPC with a key  set to `manual`. It is used to apply a network configuration specified by local user through TUI. `NIM` service has a special handling of `manual` DPC
+* `SetServer` - updates server URL in `/config/server` file. The request fails if the node is already onboarded.
+
+## Request/response representation
+
+All requests and responses are sent in JSON format. Some internal EVE structures e.g. DPCList are serialized into JSON as-is and deserialized on the rust application side.
+It introduces a problem in case a structure is updated on EVE side, but the rust application is not updated.
+To avoid this problem a proxy structures should be created  on EVE side in future.

pkg/pillar/docs/nim.md

@@ -22,7 +22,7 @@ to move to the most recent, aka the highest-priority configuration.
   * DPC is received from different sources, such as zedagent (bootstrap config
     or config from the controller), the `/config` partition with `override.json`,
     specially formatted USB stick with `usb.json` and even from NIM itself,
-    which builds and publishes the *last-resort* config if enabled
+    which builds and publishes the *last-resort* config if enabled. If `monitor` application is enabled the user can also change current network settings manually from the local TUI. In this case [monitor](./monitor.md) service sets a DPC with a key set to `manual`. Only one instance of the DPC with key `manual` may exists at a given time thus it is always overwritten when the user changes network settings
 * global configuration properties
   * an instance of `ConfigItemValueMap` struct received from zedagent
   * used to determine if last-resort should be enabled, also to obtain time