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Serial port architecture in Marlin

Marlin is targeting a plethora of different CPU architectures 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 2.0.8, 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 the CALL_IF_EXISTS macro in Marlin/src/core/macros.h for documentation of this technique.

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 create a single serial interface with 2 serial outputs (one enabled at runtime and the other switchable):

typedef MultiSerial< RuntimeSerial<Serial>, ConditionalSerial<TelnetClient> > Serial1Class;

To send the same output to 4 serial ports you could nest MultiSerial like this:

typedef MultiSerial< MultiSerial< BaseSerial<Serial>, BaseSerial<Serial1> >, MultiSerial< BaseSerial<Serial2>, BaseSerial<Serial3>, 2, 1>, 0, 2> Serial1Class;

The magical numbers here are the step and offset for computing the serial port. Simplifying the above monster a bit:

MS< A = MS<a, b, offset=0, step=1>, B=MS<c, d, offset=2, step=1>, offset=0, step=2>

This means that the underlying multiserial A (with output to a,b) is available from offset = 0 to offset + step = 1 (default value). The multiserial B (with output to c,d) is available from offset = 2 (the next step from the root multiserial) to offset + step = 3. In practice, the root multiserial will redirect any index/mask offset to offset + step - 1 to its first leaf, and any index/mask offset + step to offset + 2*step - 1 to its second leaf.

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 types take a first bool emergencyParserEnabled argument to their constructor. You must take into account this parameter when defining the actual type used.

SERIAL macros

The following macros are defined (in serial.h) to output data to the serial ports:

MACRO Parameters Usage Example Expected output
SERIAL_ECHO Any basic type is supported (char, uint8_t, int16_t, int32_t, float, long, const char*, ...). For a numeric type it prints the number in decimal. A string is output as a string. uint8_t a = 123; SERIAL_ECHO(a); SERIAL_CHAR(' '); SERIAL_ECHO(' '); 123 32
SERIAL_ECHOLN Same as SERIAL_ECHO Do SERIAL_ECHO, adding a newline int a = 456; SERIAL_ECHOLN(a); 456\n
SERIAL_ECHOPGM String / Value pairs Print a series of string literals and values alternately SERIAL_ECHOPGM("Bob", 34); Bob34
SERIAL_ECHOLNPGM Same as SERIAL_ECHOPGM Do SERIAL_ECHOPGM, adding a newline SERIAL_ECHOPGM("Alice", 56); alice56
SERIAL_ECHOPGM_P Like SERIAL_ECHOPGM but takes PGM strings Print a series of PGM strings and values alternately SERIAL_ECHOPGM_P(GET_TEXT(MSG_HELLO), 123); Hello123
SERIAL_ECHOLNPGM_P Same as SERIAL_ECHOPGM_P Do SERIAL_ECHOPGM_P, adding a newline SERIAL_ECHOLNPGM_P(PSTR("Alice"), 78); alice78\n
SERIAL_ECHO_START None Prefix an echo line SERIAL_ECHO_START(); echo:
SERIAL_ECHO_MSG Same as SERIAL_ECHOLNPGM Print a full echo line SERIAL_ECHO_MSG("Count is ", count); echo:Count is 3
SERIAL_ERROR_START None Prefix an error line SERIAL_ERROR_START(); Error:
SERIAL_ERROR_MSG Same as SERIAL_ECHOLNPGM Print a full error line SERIAL_ERROR_MSG("Not found"); Error:Not found
SERIAL_ECHO_SP Number of spaces Print one or more spaces SERIAL_ECHO_SP(3)
SERIAL_EOL None Print an end of line SERIAL_EOL(); \n

This document was written by X-Ryl669 and is under CC-SA license