Messages in this thread | | | Date | Thu, 20 Mar 2014 19:05:35 -0400 | From | Peter Hurley <> | Subject | Re: Possible software flow problem in serial_core |
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[ +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
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