CDC data lost/truncated with consecutive full packet writes

[samspencer5991]

Operating System

Linux

Board

Custom STM32G491 board

Firmware

size_t CustomWriter::flush()
{
	size_t num = tud_cdc_n_write(0, txBuf, numBytes);
	tud_cdc_n_write_flush(0);
	numBytes = 0;
	return num;
}

size_t CustomWriter::writeBuffer(const uint8_t *buffer, size_t length)
{
               ... 
               // Copy as many bytes as will fit into the buffer and transmit
		uint16_t remainingBytes = 64 - numBytes;
		memcpy(&txBuf[numBytes], modBuffer, remainingBytes);
		tud_cdc_n_write(0, txBuf, 64);
		tud_task();
		writeCount = 64;
		totalBytes -= remainingBytes;
               ...
}

What happened ?

When transferring consecutive full speed packets, some data at the end is lost unless I call tud_task() directly after tud_cdc_n_write().

For context, I am serialising some JSON data, and implementing the above writing function to buffer outgoing data into ful 64-byte packets, before sending any remaining data using the flush function. Everything is working well, however, when transmitting consecutive data packets consisting of several 64-byte packets, and a non-full packet to finish, the data is cut-off before the end (before the last non-full packet).

I can fix this by increasing the TX_CDC buffer size in my tusb_config.h file, OR adding a tud_task(); call directly in my writing function (less than ideal). But it seems I'm perhaps missing something important here.

This is my first project using TinyUSB after transitioning from the ST STM32 USB device middleware drivers, so I'm still learning the intricacies of TinyUSB.

How to reproduce ?

Difficult to provide steps here due to being a custom board and firmware. Likewise, debug info is tricky as there is no available UART port or SWO pin on this current hardware version.

Debug Log as txt file (LOG/CFG_TUSB_DEBUG=2)

no-debug-info.txt

Screenshots

No response

I have checked existing issues, dicussion and documentation

• I confirm I have checked existing issues, dicussion and documentation.