doc: Add documentation about target structure/architecture

doc/images/in-process.drawio

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doc/mainpage.md

@@ -9,6 +9,7 @@ getting started information and general documentation.
 * @ref md_CHANGELOG - If this is the web version of the docs, the changelog
 also includes a section for changes that have not yet been in a tagged
 release.
+* @ref md_targets
 * @ref md_writing-a-new-driver (**not complete**)
 
 ## Source layout

doc/targets.md

@@ -0,0 +1,73 @@
+# Understanding and Writing Targets
+
+Monado is designed to be a collection of related but independent modules. The
+final build product that brings all the desired components together, potentially
+with additional code, is called the "target". There are several targets included
+in the Monado source tree (in `src/xrt/targets/`) including:
+
+- `cli` - builds `monado-cli` executable
+- `openxr` - builds `libopenxr-monado.so` OpenXR runtime shared object
+- `gui` - builds `monado-gui` executable
+- `service` - builds `monado-service` executable (if `XRT_FEATURE_SERVICE` is
+  enabled)
+
+There is also a directory `common` which builds two static libraries. Because
+the "target" is responsible for pulling in all the desired drivers, etc. it can
+lead to some repetition if multiple targets want the same driver collection. For
+this reason, the "all drivers" code shared between many targets is located here,
+though you could consider it a part of the individual targets.
+
+## Requirements of a Target
+
+A target must first provide the entry point desired: `int main()` if it's an
+executable, or the well-known symbol name if it's a shared library. In some
+cases, the entry point might be provided by one of the modules being combined to
+form the target. For instance, an OpenXR runtime must expose
+`xrNegotiateLoaderRuntimeInterface`: this function is provided by the OpenXR
+state tracker `st_oxr`, so the OpenXR runtime target just has to link the state
+tracker in and ensure it is present in the final build product.
+
+Then, the target must provide access to the collection of devices desired. The
+code is currently (26-May-2020) in the middle of a transition, from a
+"prober"-centric interface to an "instance"-centric interface.
+
+- Target device access was historically done by implementing
+  `xrt_prober_create`, typically through a call to
+  `xrt_prober_create_with_lists`, often passing the target list defined in the
+  common `target_lists.c` shared file. This is currently still somewhat-required
+  but not as central as before, and will be removed soon.
+- Target device access is now provided by implementing the `xrt_instance`
+  interface in your target and providing a definition of `xrt_instance_create`
+  that instantiates your implementation.
+
+All methods of `xrt_instance` are required, though the `get_prober` method may
+output a null pointer if the instance is not using a prober. For more detailed
+information on this interface, see the documentation for @ref xrt_instance
+
+## Sample Call Trees
+
+For clarity, call trees are included below for the OpenXR runtime in two general
+cases: `XRT_FEATURE_SERVICE` disabled, and `XRT_FEATURE_SERVICE` enabled. Note
+that even with `XRT_FEATURE_SERVICE` enabled, the other targets (cli, gui) more
+closely resembler the `XRT_FEATURE_SERVICE` disabled diagram: they contain the
+device drivers internally rather than contacting the service.
+
+### XRT_FEATURE_SERVICE disabled
+
+This is the simplest architecture. It is also the architecture used by the
+various extra targets like `monado-cli` even when building with
+`XRT_FEATURE_SERVICE` enabled.
+
+![In-process OpenXR runtime diagram](images/in-process.svg)
+
+### XRT_FEATURE_SERVICE enabled
+
+Note that in this case, there are two processes involved, which have different
+`xrt_instance` implementations.
+
+- The runtime has a "stub" or "client proxy" implementation that delegates to
+  the service over the IPC.
+- The service has a normal or complete instance implementation that actually
+  provides access to devices, etc.
+
+![Out-of-process OpenXR runtime diagram](images/out-of-proc.svg)