KernelTimer mystery using USBDevice

[u77345]

What could be the reason the write to USBSerial device from kernel timer callback locks up?
The assertions pass, and the logger correctly finds and prints m_magic as expected. However, when I attempt to write the CDevice (CUSBSerial), the application locks up. What could be causing this, how can I further diagnose it?

void CFrontPanelMenu::TransitionTimerHandler(TKernelTimerHandle hTimer, void* pParam, void* pContext)
{
    CFrontPanelMenu *pThis = (CFrontPanelMenu *) pContext;
    assert(pThis != 0);
    assert(pThis->m_pFrontPanel != 0);
    assert(pThis->m_pFrontPanel->m_pFrontPanelDevice != 0);

    pThis->m_displayState = DISPLAY_NORMAL;
    CLogger::Get()->Write("frontPanelMenu", LogError, "Timer %s", pThis->m_magic);
    //pThis->UpdateDisplay(); //CRASHES
    //pThis->m_pFrontPanel->frontPanelDisplayCentered("TIMER");  // CRASHES
    pThis->m_pFrontPanel->m_pFrontPanelDevice->Write("DTIMER\n\r", 8);  // CRASHES
}

[rsta2]

A kernel timer handler is called at IRQ_LEVEL from the system timer IRQ handler. At this level other IRQs are disabled, as long the kernel timer handler runs. Writing to the a CUSBSerialDevice is a complex operation, which includes steps, where the USB IRQ handler has to be called. Because IRQs are forbidden, the USB driver waits forever for the USB IRQ and the system hangs. The conclusion is, that you normally cannot write to a CUSBSerialDevice from a kernel timer handler. There's a solution, if you must write to a CUSBSerial device from a kernel timer handler. You can use the class CWriteBufferDevice to buffer the writes in the timer handler and complete the writes in your application's main loop at TASK_LEVEL by calling CWriteBufferDevice::Update(). Please have a look at sample/40-irqlatency, which uses the class CWriteBufferDevice to buffer writes to CScreenDevice. You can use CUSBSerialDevice similar to CScreenDevice in this sample.

[u77345]

Thanks, Rene! What an elegant solution! It only took two prompts to Claude in Cursor to integrate.