I have been trying to receive a 62420 byte packet over a USB virtual serial port (STM32 microcontroller in CDC mode). As such data is transmitted at the maximum USB Full Speed rate, around 1MB/sec. The entire packet is sent via DMA in one go, once per second.

I wrote a trivial program to test this, and found that some bytes are randomly dropped.

The core of the test app is


uint8_t buffer[65536];
for(;;)
{
	int bytes_read = sp_blocking_read(port, buffer, sizeof(buffer), 500);
	if (bytes_read > 0)
		printf("Read %d\n", bytes_read);
}

I would expect this to read one entire packet. The output is as follows:


Read 60971
Read 60968
Read 61005
Read 60977
Read 60859
Read 60917
Read 60872
Read 60913
Read 60895
Read 60859

There are some potential timing issues here with the 500ms timeout, but I have verified with 250ms and 100ms time-outs that in all cases at least some data is lost.

This only seems to affect Linux, which I am running on a Raspberry Pi. On Windows the code works perfectly and I get

Read 62420
Read 62420
Read 62420

So it appears that on Linux the OS is not buffering the data so much. I tried using non-blocking reads instead:


for(;;)
{
	int bytes_read = sp_nonblocking_read(port, buffer, sizeof(buffer));
	if (bytes_read > 0)
		printf("Read %d\n", bytes_read);
}


The output was as follows:

...
Read 128
Read 127
Read 128
Read 128
Read 128
Read 84

It appears that occasionally bytes are dropped from whatever 128 byte buffer this is, probably the OS as I can't find references to that value in libserialport.

I'm not sure what the best resolution would be. Can the buffer be increased? 128 bytes is rather low for USB serial ports that aren't emulating the baud rate.