Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753220AbdFGXFE (ORCPT ); Wed, 7 Jun 2017 19:05:04 -0400 Received: from wtarreau.pck.nerim.net ([62.212.114.60]:50671 "EHLO 1wt.eu" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753183AbdFGXFA (ORCPT ); Wed, 7 Jun 2017 19:05:00 -0400 From: Willy Tarreau To: linux-kernel@vger.kernel.org, stable@vger.kernel.org, linux@roeck-us.net Cc: Johan Hovold , Willy Tarreau Subject: [PATCH 3.10 128/250] USB: serial: ftdi_sio: fix extreme low-latency setting Date: Thu, 8 Jun 2017 00:58:34 +0200 Message-Id: <1496876436-32402-129-git-send-email-w@1wt.eu> X-Mailer: git-send-email 2.8.0.rc2.1.gbe9624a In-Reply-To: <1496876436-32402-1-git-send-email-w@1wt.eu> References: <1496876436-32402-1-git-send-email-w@1wt.eu> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1838 Lines: 49 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: Willy Tarreau --- drivers/usb/serial/ftdi_sio.c | 2 -- 1 file changed, 2 deletions(-) diff --git a/drivers/usb/serial/ftdi_sio.c b/drivers/usb/serial/ftdi_sio.c index ba04308..960d0e4 100644 --- a/drivers/usb/serial/ftdi_sio.c +++ b/drivers/usb/serial/ftdi_sio.c @@ -1813,8 +1813,6 @@ static int ftdi_sio_port_probe(struct usb_serial_port *port) mutex_init(&priv->cfg_lock); - priv->flags = ASYNC_LOW_LATENCY; - if (quirk && quirk->port_probe) quirk->port_probe(priv); -- 2.8.0.rc2.1.gbe9624a