From mboxrd@z Thu Jan 1 00:00:00 1970 From: Peter Hurley Subject: Re: Possible software flow problem in serial_core Date: Thu, 20 Mar 2014 19:05:35 -0400 Message-ID: <532B743F.2030705@hurleysoftware.com> References: Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: QUOTED-PRINTABLE Return-path: Received: from mailout32.mail01.mtsvc.net ([216.70.64.70]:53910 "EHLO n23.mail01.mtsvc.net" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1759160AbaCTXFh (ORCPT ); Thu, 20 Mar 2014 19:05:37 -0400 In-Reply-To: Sender: linux-serial-owner@vger.kernel.org List-Id: linux-serial@vger.kernel.org To: "Bollinger, Seth" Cc: "linux-kernel@vger.kernel.org" , linux-serial [ +cc linux-serial ] On 03/20/2014 09:44 AM, Bollinger, Seth wrote: > Hello All, > > We=E2=80=99ve 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 a= s the > serial_core ring is cleared (this could trap a few bytes in the fifo)= =2E > 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 unti= l > 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 th= is > eventuality? > > Thanks, > > Seth What hardware is this? A 16550 transmitter doesn't shut-off at uart->stop_tx(), so the remaini= ng 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 wit= hout 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@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html