Translate Mackie Control in the dongle: a freestanding C module, shared with the firmware

.gitignore

@@ -1,4 +1,5 @@
 /build/
+/build-*/
 *.o
 *.a
 compile_commands.json

CMakeLists.txt

@@ -1,9 +1,11 @@
 # SPDX-License-Identifier: GPL-3.0-or-later
 cmake_minimum_required(VERSION 3.16)
-project(command8 LANGUAGES CXX)
+project(command8 LANGUAGES C CXX)
 
 set(CMAKE_CXX_STANDARD 17)
 set(CMAKE_CXX_STANDARD_REQUIRED ON)
+set(CMAKE_C_STANDARD 99)
+set(CMAKE_C_STANDARD_REQUIRED ON)
 if(NOT CMAKE_BUILD_TYPE AND NOT CMAKE_CONFIGURATION_TYPES)
   set(CMAKE_BUILD_TYPE Release)
 endif()
@@ -28,6 +30,15 @@ add_library(command8 STATIC
 target_include_directories(command8 PUBLIC src)
 target_compile_options(command8 PRIVATE ${C8_WARNINGS})
 
+# Freestanding C99 Command|8 <-> Mackie Control translator, shared verbatim with
+# the dongle firmware, where it runs on bare metal with no allocator and no C++
+# runtime. It is a separate target with no link dependencies on purpose: the
+# build then enforces that property instead of leaving it to convention, so an
+# accidental #include of a host header fails here rather than on the RP2040.
+add_library(command8-mcu STATIC src/mcu/c8_mcu.c)
+target_include_directories(command8-mcu PUBLIC src)
+target_compile_options(command8-mcu PRIVATE ${C8_WARNINGS})
+
 if(WIN32)
   find_package(RtMidi CONFIG REQUIRED)
   target_sources(command8 PRIVATE
@@ -162,6 +173,11 @@ target_link_libraries(test_protocol PRIVATE command8)
 target_compile_options(test_protocol PRIVATE ${C8_WARNINGS})
 add_test(NAME protocol COMMAND test_protocol)
 
+add_executable(test_mcu tests/test_mcu.cpp)
+target_link_libraries(test_mcu PRIVATE command8 command8-mcu)
+target_compile_options(test_mcu PRIVATE ${C8_WARNINGS})
+add_test(NAME mcu COMMAND test_mcu)
+
 add_executable(test_feedback tests/test_feedback.cpp)
 target_link_libraries(test_feedback PRIVATE command8)
 target_compile_options(test_feedback PRIVATE ${C8_WARNINGS})

README.md

@@ -204,19 +204,44 @@ talks to the engine (over OSC, or over the MCU loopback pair), never to the
 surface directly: the Command|8 speaks a proprietary protocol, so a DAW sending
 it generic MIDI just makes the faders twitch and leaves the display Offline.
 
+### No bridge at all: the dongle's MCU cable
+
+A dongle built with on-board translation exposes a fourth cable that *is* a
+Mackie Control endpoint, so none of the above is needed — no engine process, no
+loopback pair, no OSC. Point the DAW's Mackie Control support straight at it,
+input and output, and leave the surface's own port disabled.
+
+The cable is named `Command8 MCU` on Linux and macOS. On Windows, WinMM names
+cables positionally and ignores the jack strings, so it appears as
+`MIDIIN4 (Command|8 Bridge)` / `MIDIOUT4 (Command|8 Bridge)` — the fourth port.
+
+Verified on hardware 2026-08-11: Reaper driving a Command|8 through the dongle
+on Windows with no host software running.
+
+The translation is the same `src/mcu/c8_mcu.c` this repo unit-tests on the
+desktop; the firmware compiles that file directly rather than a copy.
+
 ### Mackie bridge on Windows
 
 Windows has no app-created virtual MIDI ports, so create a loopback pair once
 with [Windows MIDI Services](https://aka.ms/midi) (or two loopMIDI cables and
 `--mcu-recv`/`--mcu-send`):
 
 ```bat
-midi loopback create --name-a "Command8 MCU A" --name-b "Command8 MCU B"
+midi loopback create --name-a "Command8 MCU Bridge" --name-b "Command8 MCU DAW"
 ```
 
-`command8-mackie` uses side **A** by default; point the DAW's Mackie Control
-input *and* output at side **B**. The pair is crossed, so neither end hears its
-own output.
+The ends are named for who owns them. `command8-mackie` takes **Command8 MCU
+Bridge** by default; point the DAW's Mackie Control input *and* output at
+**Command8 MCU DAW**. The pair is crossed, so neither end hears its own output.
+
+Note there is no bar in these names, deliberately. `Command|8` is how the tools
+find the surface itself, by prefix, so a loopback named `Command|8 MCU …` could
+be matched as the device. Keep the loopback pair on the unbarred `Command8`.
+
+Verified end to end on Windows: with the three `Command|8 Bridge` device ports
+left **disabled** in Reaper's MIDI Devices, `Command8 MCU DAW` enabled, and a
+Mackie Control Universal surface pointed at it for both input and output.
 
 
 ## License

src/mackie/mackie_backend.hpp

@@ -21,16 +21,26 @@
 namespace command8 {
 
 // Default MCU port-name matches. recv = DAW-to-bridge, send = bridge-to-DAW.
-// Linux: one duplex virmidi port carries both directions. Windows: side A of a
-// Windows MIDI Services loopback pair (create once with
-//   midi loopback create --name-a "Command8 MCU A" --name-b "Command8 MCU B"
-// ); the bridge opens A both ways and the DAW's Mackie Control uses B, so
-// neither hears its own output. macOS: these are the names of the virtual
-// ports the bridge CREATES rather than ones to search for, so no loopback is
-// needed and the DAW points at this name for both directions.
+// Linux: one duplex virmidi port carries both directions. Windows: the bridge
+// end of a Windows MIDI Services loopback pair (create once with
+//   midi loopback create --name-a "Command8 MCU Bridge" --name-b "Command8 MCU DAW"
+// ); the bridge opens its own end both ways and the DAW's Mackie Control uses
+// the other, so neither hears its own output. macOS: these are the names of the
+// virtual ports the bridge CREATES rather than ones to search for, so no
+// loopback is needed and the DAW points at this name for both directions.
+//
+// The ends are named for who owns them because "A" and "B" gave no clue which
+// was which, and picking the wrong one in the DAW produces a silent failure
+// that looks exactly like broken hardware.
+//
+// Do NOT put a bar in these names. "Command|8" is the surface port matcher (see
+// kDefaultPortMatch in surface.hpp), and it takes the first prefix match it
+// finds -- a loopback called "Command|8 MCU ..." could win that match ahead of
+// the real device depending on enumeration order. The unbarred "Command8" is
+// what keeps the two families of port distinguishable.
 #if defined(_WIN32)
-inline constexpr const char* kDefaultMcuRecvMatch = "Command8 MCU A";
-inline constexpr const char* kDefaultMcuSendMatch = "Command8 MCU A";
+inline constexpr const char* kDefaultMcuRecvMatch = "Command8 MCU Bridge";
+inline constexpr const char* kDefaultMcuSendMatch = "Command8 MCU Bridge";
 #elif defined(__APPLE__)
 inline constexpr const char* kDefaultMcuRecvMatch = "Command|8";
 inline constexpr const char* kDefaultMcuSendMatch = "Command|8";

src/mackie/mackie_main.cpp

@@ -5,8 +5,11 @@
 // Linux: use a snd-virmidi kernel port so DAWs (e.g. Bitwig) can see it:
 //   sudo modprobe snd-virmidi   -> "Virtual Raw MIDI 4-0..4-3"
 // Windows: create a Windows MIDI Services loopback pair once:
-//   midi loopback create --name-a "Command8 MCU A" --name-b "Command8 MCU B"
-// The bridge uses side A; point the DAW's Mackie Control at side B (in + out).
+//   midi loopback create --name-a "Command8 MCU Bridge" --name-b "Command8 MCU DAW"
+// The bridge takes the "Bridge" end; point the DAW's Mackie Control at the
+// "DAW" end (in + out). Leave the "Command|8 Bridge" device ports themselves
+// disabled in the DAW -- those belong to this process, and a DAW holding them
+// stops it opening the surface at all.
 //
 //   ./command8-mackie [--mcu-port <match>] [--mcu-recv <match>]
 //                     [--mcu-send <match>] [--port <device-match>]
@@ -83,8 +86,8 @@ int main(int argc, char** argv) {
 #ifdef _WIN32
         std::fprintf(stderr, "Could not open the MCU ports ('%s' / '%s'). Create "
                      "the loopback pair first:\n  midi loopback create --name-a "
-                     "\"Command8 MCU A\" --name-b \"Command8 MCU B\"\n(or create "
-                     "cables in loopMIDI) and retry.\n",
+                     "\"Command8 MCU Bridge\" --name-b \"Command8 MCU DAW\"\n"
+                     "(or create cables in loopMIDI) and retry.\n",
                      recv_match.c_str(), send_match.c_str());
 #else
         std::fprintf(stderr, "Could not open an MCU port matching '%s'. Load "