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+# Serial port architecture in Marlin
+
+Marlin is targeting a pletora of different CPU architecture and platforms. Each of these platforms has its own serial interface.
+While many provide a Arduino-like Serial class, it's not all of them, and the differences in the existing API create a very complex brain teaser for writing code that works more or less on each platform.
+
+Moreover, many platform have intrinsic needs about serial port (like forwarding the output on multiple serial port, providing a *serial-like* telnet server, mixing USB-based serial port with SD card emulation) that are difficult to handle cleanly in the other platform serial logic.
+
+
+Starting with version `2.0.9`, Marlin provides a common interface for its serial needs.
+
+## Common interface
+
+This interface is declared in `Marlin/src/core/serial_base.h`
+Any implementation will need to follow this interface for being used transparently in Marlin's codebase.
+
+The implementation was written to prioritize performance over abstraction, so the base interface is not using virtual inheritance to avoid the cost of virtual dispatching while calling methods.
+Instead, the Curiously Recurring Template Pattern (**CRTP**) is used so that, upon compilation, the interface abstraction does not incur a performance cost.
+
+Because some platform do not follow the same interface, the missing method in the actual low-level implementation are detected via SFINAE and a wrapper is generated when such method are missing. See `CALL_IF_EXISTS` macro in `Marlin/src/core/macros.h` for the documentation of this technic.
+
+## Composing the desired feature
+The different specificities for each architecture are provided by composing the serial type based on desired functionality.
+In the `Marlin/src/core/serial_hook.h` file, the different serial feature are declared and defined in each templated type:
+1. `BaseSerial` is a simple 1:1 wrapper to the underlying, Arduino compatible, `Serial`'s class. It derives from it. You'll use this if the platform does not do anything specific for the `Serial` object (for example, if an interrupt callback calls directly the serial **instance** in the platform's framework code, this is not the right class to use). This wrapper is completely inlined so that it does not generate any code upon compilation. `BaseSerial` constructor forwards any parameter to the platform's `Serial`'s constructor.
+2. `ForwardSerial` is a composing wrapper. It references an actual Arduino compatible `Serial` instance. You'll use this if the instance is declared in the platform's framework and is being referred directly in the framework. This is not as efficient as the `BaseSerial` implementation since static dereferencing is done for each method call (it'll still be faster than virtual dispatching)
+3. `ConditionalSerial` is working a bit like the `ForwardSerial` interface, but it checks a boolean condition before calling the referenced instance. You'll use it when the serial output can be switch off at runtime, for example in a *telnet* like serial output that should not emit any packet if no client is connected.
+4. `RuntimeSerial` is providing a runtime-modifiable hooking method for its `write` and `msgDone` method. You'll use it if you need to capture the serial output of Marlin, for example to display the G-Code parser's output on a GUI interface. The hooking interface is setup via the `setHook` method.
+5. `MultiSerial` is a runtime modifiable serial output multiplexer. It can output (*respectively input*) to 2 different interface based on a port *mask*. You'll use this if you need to output the same serial stream to multiple port. You can plug a `MultiSerial` to itself to duplicate to more than 2 ports.
+
+## Plumbing
+Since all the types above are using CRTP, it's possible to combine them to get the appropriate functionality.
+This is easily done via type definition of the feature.
+
+For example, to present a serial interface that's outputting to 2 serial port, the first one being hooked at runtime and the second one connected to a runtime switchable telnet client, you'll declare the type to use as:
+```
+typedef MultiSerial< RuntimeSerial<Serial>, ConditionalSerial<TelnetClient> > Serial0Type;
+```
+
+## Emergency parser
+By default, the serial base interface provide an emergency parser that's only enable for serial classes that support it.
+Because of this condition, all underlying type takes a first `bool emergencyParserEnabled` argument to their constructor. You must take into account this parameter when defining the actual type used.
+
+
+*This document was written by [X-Ryl669](https://blog.cyril.by) and is under [CC-SA license](https://creativecommons.org/licenses/by-sa)*