72 lines
7.2 KiB
Markdown
72 lines
7.2 KiB
Markdown
# Serial port architecture in Marlin
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Marlin is targeting a plethora of different CPU architectures and platforms. Each of these platforms has its own serial interface.
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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.
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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.
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Starting with version 2.0.8, Marlin provides a common interface for its serial needs.
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## Common interface
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This interface is declared in `Marlin/src/core/serial_base.h`
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Any implementation will need to follow this interface for being used transparently in Marlin's codebase.
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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.
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Instead, the Curiously Recurring Template Pattern (**CRTP**) is used so that, upon compilation, the interface abstraction does not incur a performance cost.
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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.
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## Composing the desired feature
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The different specificities for each architecture are provided by composing the serial type based on desired functionality.
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In the `Marlin/src/core/serial_hook.h` file, the different serial feature are declared and defined in each templated type:
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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.
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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)
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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.
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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.
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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.
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## Plumbing
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Since all the types above are using CRTP, it's possible to combine them to get the appropriate functionality.
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This is easily done via type definition of the feature.
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For example, to create a single serial interface with 2 serial outputs (one enabled at runtime and the other switchable):
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```cpp
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typedef MultiSerial< RuntimeSerial<Serial>, ConditionalSerial<TelnetClient> > Serial1Class;
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```
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To send the same output to 4 serial ports you could nest `MultiSerial` like this:
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```cpp
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typedef MultiSerial< MultiSerial< BaseSerial<Serial>, BaseSerial<Serial1> >, MultiSerial< BaseSerial<Serial2>, BaseSerial<Serial3>, 2, 1>, 0, 2> Serial1Class;
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```
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The magical numbers here are the step and offset for computing the serial port. Simplifying the above monster a bit:
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```cpp
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MS< A = MS<a, b, offset=0, step=1>, B=MS<c, d, offset=2, step=1>, offset=0, step=2>
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```
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This means that the underlying multiserial A (with output to `a,b`) is available from offset = 0 to offset + step = 1 (default value).
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The multiserial B (with output to `c,d`) is available from offset = 2 (the next step from the root multiserial) to offset + step = 3.
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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.
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## Emergency parser
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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.
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## SERIAL macros
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The following macros are defined (in `serial.h`) to output data to the serial ports:
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| MACRO | Parameters | Usage | Example | Expected output |
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|-------|------------|-------|---------|-----------------|
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| `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` |
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| `SERIAL_ECHOLN` | Same as `SERIAL_ECHO` | Do `SERIAL_ECHO`, adding a newline | `int a = 456; SERIAL_ECHOLN(a);` | `456\n` |
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| `SERIAL_ECHOPGM` | String / Value pairs | Print a series of string literals and values alternately | `SERIAL_ECHOPGM("Bob", 34);` | `Bob34` |
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| `SERIAL_ECHOLNPGM` | Same as `SERIAL_ECHOPGM` | Do `SERIAL_ECHOPGM`, adding a newline | `SERIAL_ECHOPGM("Alice", 56);` | `alice56` |
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| `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` |
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| `SERIAL_ECHOLNPGM_P` | Same as `SERIAL_ECHOPGM_P` | Do `SERIAL_ECHOPGM_P`, adding a newline | `SERIAL_ECHOLNPGM_P(PSTR("Alice"), 78);` | `alice78\n` |
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| `SERIAL_ECHO_START` | None | Prefix an echo line | `SERIAL_ECHO_START();` | `echo:` |
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| `SERIAL_ECHO_MSG` | Same as `SERIAL_ECHOLNPGM` | Print a full echo line | `SERIAL_ECHO_MSG("Count is ", count);` | `echo:Count is 3` |
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| `SERIAL_ERROR_START`| None | Prefix an error line | `SERIAL_ERROR_START();` | `Error:` |
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| `SERIAL_ERROR_MSG` | Same as `SERIAL_ECHOLNPGM` | Print a full error line | `SERIAL_ERROR_MSG("Not found");` | `Error:Not found` |
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| `SERIAL_ECHO_SP` | Number of spaces | Print one or more spaces | `SERIAL_ECHO_SP(3)` | ` ` |
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| `SERIAL_EOL` | None | Print an end of line | `SERIAL_EOL();` | `\n` |
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*This document was written by [X-Ryl669](https://blog.cyril.by) and is under [CC-SA license](https://creativecommons.org/licenses/by-sa)*
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