[Epic] - LekaOS Coordination

[ladislas]

This epic will discuss how we should organize the inner working of LekaOS.

[ladislas]

State Machines

• Whitepaper-Statemachines.pdf

[ladislas]

Watchdog

Compare between ticker & thread

• Related tasks: [Task] - Watchdog - Compare between thread and ticker #48

Different things are to be compared:

• build size
• simplicity of code
• handling of abort()
• handling of infinite loops
• interference with deep sleep

Build size

• with thread: 34980 bytes
• with ticker: 33300 bytes
• difference: thread - ticker = 1680 bytes

The use of a ticker instead of a thread saves 1680 bytes of memory.

🏅 for ticker.

$ cmake --build build -t spike_mbed_watchdog_ticker_vs_thread

[2/2] Linking CXX executable spikes/mbed_watchdog_ticker_vs_thread/spike_mbed_watchdog_ticker_vs_thread
| Module                 |         .text |       .data |        .bss |
|------------------------|---------------|-------------|-------------|
| [fill]                 |       60(+60) |       4(+4) |     32(+32) |
| [lib]/c.a              |   4852(+4852) | 2108(+2108) |     89(+89) |
| [lib]/gcc.a            |     760(+760) |       0(+0) |       0(+0) |
| [lib]/mbed-os-static.a | 24520(+24520) |   444(+444) | 8050(+8050) |
| [lib]/misc             |     188(+188) |       4(+4) |     28(+28) |
| main.cpp.obj           |     360(+360) |       0(+0) |   129(+129) |
| Subtotals              | 30740(+30740) | 2560(+2560) | 8328(+8328) |
Total Static RAM memory (data + bss): 10888(+10888) bytes
Total Flash memory (text + data): 33300(+33300) bytes

$ cmake --build build -t spike_mbed_watchdog_ticker_vs_thread

[2/2] Linking CXX executable spikes/mbed_watchdog_ticker_vs_thread/spike_mbed_watchdog_ticker_vs_thread
| Module                 |         .text |       .data |        .bss |
|------------------------|---------------|-------------|-------------|
| [fill]                 |       64(+64) |       4(+4) |     28(+28) |
| [lib]/c.a              |   4852(+4852) | 2108(+2108) |     89(+89) |
| [lib]/gcc.a            |     760(+760) |       0(+0) |       0(+0) |
| [lib]/mbed-os-static.a | 26176(+26176) |   444(+444) | 8050(+8050) |
| [lib]/misc             |     188(+188) |       4(+4) |     28(+28) |
| main.cpp.obj           |     380(+380) |       0(+0) |   253(+253) |
| Subtotals              | 32420(+32420) | 2560(+2560) | 8448(+8448) |
Total Static RAM memory (data + bss): 11008(+11008) bytes
Total Flash memory (text + data): 34980(+34980) bytes

Simplicity of code

See spikes/mbed_watchdog_ticker_vs_thread/main.cpp.

The ticker simpler as you only need a void kick() function to attach, whereas the thread also need a thread function to be started.

🏅 for ticker

Handling of abort()

Both ticker and thread can handle an abort() and thus a fatal error to restart the robot.

Handling of infinite loop

As a surprise, an infinite loop doesn't stop the rtos from running and the watchdog is kicked periodically for both ticker and thread.

After investigation, the reason comes from their respective priority:

• the ticker is triggered by interrupt, thus is will always have the highest priority and preempt any already running tasks, even in a while(true){} loop
• a thread is created with osPriorityNormal. If both the infinite loop thread and the watchdog thread have the same priority or if the watchdog thread has a higher priority, it will preempt the infinite loop and kick the watchdog. In this case, it's not possible to catch an infinite loop and reset the system if it happens.

The watchdog thread priority can be modified with set_priority (osPriority priority) once the thread is running and set to osPriorityLow. That way, all tasks have a higher priority than the watchdog. If one task gets stuck, the watchdog won't be kicked and the system will reset.

⚠️ It's important to note that this is true for blocking infinite loops, without wait() functions or with busy wait functions (HAL_Delay or wait_ns). If the infinite loop uses ThisThread::sleep_for(), the watchdog will be kicked and the system will stay stucked.

In this case, a reset command should be sent to the system through BLE.

🏅 for thread