[ +cc linux-serial ] On 03/20/2014 09:44 AM, Bollinger, Seth wrote:
Hello All, We’ve recently run into a possible problem with software flow control handling in the serial_core layer. Here is the scenario: 1. Transmit from uart to remote device 2. Remote device sends us an XOFF 3. The tty layer receives the XOFF 4. stop_tty() - The uart transmitter is stopped (ops->stop_tx) just as the serial_core ring is cleared (this could trap a few bytes in the fifo). 5. Remote device sends us an XON 6. The tty layer receives the XON 7. start_tty() - However, the serial_core ring is empty, so the call to start the uart transmitter (uart_start:ops->start_tx) is skipped Any window, however small, could leave bytes stuck in the transmitter forever -- particularly if there will be no further transmission until receiving a response. I can't find any functionality in the drivers that accounts for this possibility. Can you help me find how Linux serial drivers manage this eventuality? Thanks, Seth
What hardware is this? A 16550 transmitter doesn't shut-off at uart->stop_tx(), so the remaining data in the transmitter FIFO is still transmitted. Some hardware has (relatively) big FIFOs and allows the transmitter to be shutoff, but these should unconditionally re-enable the transmitter without clearing the xmit fifo. Are you seeing lost data? Maybe it's because soft flow-control is to slow to shutoff the sender? Regards, Peter Hurley -- To unsubscribe from this list: send the line "unsubscribe linux-serial" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
