From 684d1b175d14b455d3a8f88da00aace451d7d516 Mon Sep 17 00:00:00 2001
From: George Kiagiadakis <george.kiagiadakis@collabora.com>
Date: Tue, 3 Aug 2021 12:13:30 +0300
Subject: add README.md

Signed-off-by: George Kiagiadakis <george.kiagiadakis@collabora.com>
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+PipeWire Instrument Cluster IPC
+===============================
+
+This software component provides a communication link between applications
+running in the Instrument Cluster container and the host container's PipeWire
+daemon. It is useful when IC applications need to suspend sound output from the
+host, temporarily, in order to play an emergency sound.
+
+This component is tailored for use in containerized environments. For use in
+an environment where the IC is a different host, additional work may be required.
+
+## libicipc
+
+Most of the functionality is implemented as a library, called `icipc`.
+
+This library implements both the server-side and the client-side.
+The server-side is meant to be used by the host, while the client-side is
+to be used by the applications running in the IC guest.
+
+The library implements a generic unix socket based communication mechanism
+where a sender (client) can send any data buffer to the receiver (server).
+The server is always required to respond.
+
+```
+  client  ------- REQUEST -------->  server
+  client  <--- REPLY (OK/ERROR) ---  server
+```
+
+The server is also able to track the clients that are connected, individually.
+When a client connects or disconnects, the server is notified.
+
+The protocol used for communication is a binary serialization of data structures,
+that aims to be compatible with PipeWire's spa_pod serialization mechanism,
+which is used in PipeWire's native protocol. The implementation of this protocol
+is kept private, for now, as it is not needed for the simple use case of sending
+suspend/resume requests (see below).
+
+## module-protocol-ic-ipc
+
+The server-side component is built as a pipewire module, called
+`libpipewire-module-protocol-ic-ipc`. It is meant to be executed on
+a standalone pipewire instance, using the pipewire configuration file provided:
+`pipewire-ic-ipc.conf`
+
+To start the server manually, execute:
+```
+$ pipewire -c /absolute/path/to/pipewire-ic-ipc.conf
+```
+
+For convenience, systemd `.service` and `.socket` units are provided.
+
+The server listens on a socket, which can be specified in the
+`pipewire-ic-ipc.conf` file as an argument to the module. When launched via
+systemd socket activation, the socket is passed on from systemd.
+
+The path of the socket is determined by the same rules as the ones that PipeWire
+uses for its socket. First, the `PIPEWIRE_RUNTIME_DIR` environment variable
+is used. Then, `XDG_RUNTIME_DIR`, followed by `HOME`.
+
+This server can accept two requests from its clients:
+
+ * `SUSPEND`: This sets the `suspend.playback` metadata key on pipewire's
+    main metadata object, which is then used by the session manager to suspend
+    playback.
+ * `RESUME`: This unsets the `suspend.playback` metadata key, reverting the
+    actions of `SUSPEND`
+
+When multiple clients send `SUSPEND`, the server keeps track of how many clients
+have done this and expects an equal amount of `RESUME` requests before unsetting
+the `suspend.playback` metadata key.
+
+When a client sends `SUSPEND` and then disconnects without sending `RESUME`,
+the server considers the disconnection event to be equivalent to a `RESUME`
+request. Therefore, clients do not explicitly need to send `RESUME` before
+disconnecting.
+
+## icipc-client
+
+This is an example client, which is provided for testing & demo purposes.
+When launched, it presents a command prompt that accepts 2 commands:
+
+ * `quit` - exits the client
+ * `send <request_name>` - sends `<request_name>` to the server
+
+To demo it with `libpipewire-module-protocol-ic-ipc` being the server, use:
+
+ * `send SUSPEND` - suspend playback on the host
+ * `send RESUME` - resume playback on the host
+
+## Compilation & Running
+
+To compile:
+```
+$ meson build
+$ ninja -C build
+```
+
+To run tests:
+```
+$ ninja -C build test
+... or ...
+$ meson test -C build
+```
+
+To perform static code analysis using clang-tidy:
+```
+$ ninja -C build clang-tidy
+```
+
+### Depedencies
+
+ * `libsystemd` is needed when building the library for the server-side
+   (host-side) in order to integrate with systemd's socket-activation mechanism
+
+No other dependencies are required. The code builds on top of the standard
+C library only.
-- 
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