Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752371AbdFAQan (ORCPT ); Thu, 1 Jun 2017 12:30:43 -0400 Received: from shadbolt.e.decadent.org.uk ([88.96.1.126]:33301 "EHLO shadbolt.e.decadent.org.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751958AbdFAPop (ORCPT ); Thu, 1 Jun 2017 11:44:45 -0400 Content-Type: text/plain; charset="UTF-8" Content-Disposition: inline Content-Transfer-Encoding: 8bit MIME-Version: 1.0 From: Ben Hutchings To: linux-kernel@vger.kernel.org, stable@vger.kernel.org CC: akpm@linux-foundation.org, "Johan Hovold" , "Antoine Aubert" Date: Thu, 01 Jun 2017 16:43:16 +0100 Message-ID: X-Mailer: LinuxStableQueue (scripts by bwh) Subject: [PATCH 3.16 074/212] USB: serial: ftdi_sio: fix extreme low-latency setting In-Reply-To: X-SA-Exim-Connect-IP: 82.70.136.246 X-SA-Exim-Mail-From: ben@decadent.org.uk X-SA-Exim-Scanned: No (on shadbolt.decadent.org.uk); SAEximRunCond expanded to false Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1797 Lines: 49 3.16.44-rc1 review patch. If anyone has any objections, please let me know. ------------------ From: Johan Hovold commit c6dce2626606ef16434802989466636bc28c1419 upstream. Since commit 557aaa7ffab6 ("ft232: support the ASYNC_LOW_LATENCY flag") the FTDI driver has been using a receive latency-timer value of 1 ms instead of the device default of 16 ms. The latency timer is used to periodically empty a non-full receive buffer, but a status header is always sent when the timer expires including when the buffer is empty. This means that a two-byte bulk message is received every millisecond also for an otherwise idle port as long as it is open. Let's restore the pre-2009 behaviour which reduces the rate of the status messages to 1/16th (e.g. interrupt frequency drops from 1 kHz to 62.5 Hz) by not setting ASYNC_LOW_LATENCY by default. Anyone willing to pay the price for the minimum-latency behaviour should set the flag explicitly instead using the TIOCSSERIAL ioctl or a tool such as setserial (e.g. setserial /dev/ttyUSB0 low_latency). Note that since commit 0cbd81a9f6ba ("USB: ftdi_sio: remove tty->low_latency") the ASYNC_LOW_LATENCY flag has no other effects but to set a minimal latency timer. Reported-by: Antoine Aubert Fixes: 557aaa7ffab6 ("ft232: support the ASYNC_LOW_LATENCY flag") Signed-off-by: Johan Hovold Signed-off-by: Ben Hutchings --- drivers/usb/serial/ftdi_sio.c | 2 -- 1 file changed, 2 deletions(-) --- a/drivers/usb/serial/ftdi_sio.c +++ b/drivers/usb/serial/ftdi_sio.c @@ -1828,8 +1828,6 @@ static int ftdi_sio_port_probe(struct us mutex_init(&priv->cfg_lock); - priv->flags = ASYNC_LOW_LATENCY; - if (quirk && quirk->port_probe) quirk->port_probe(priv);