From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-lf0-f66.google.com ([209.85.215.66]:35176 "EHLO mail-lf0-f66.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751262AbdAYOfx (ORCPT ); Wed, 25 Jan 2017 09:35:53 -0500 From: Johan Hovold To: linux-usb@vger.kernel.org, Greg Kroah-Hartman Cc: Antoine Aubert , stable@vger.kernel.org, Johan Hovold Subject: [PATCH] USB: serial: ftdi_sio: fix extreme low-latency setting Date: Wed, 25 Jan 2017 15:35:20 +0100 Message-Id: <20170125143520.10982-1-johan@kernel.org> Sender: stable-owner@vger.kernel.org List-ID: 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 rate of 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") Cc: stable # v2.6.31 Signed-off-by: Johan Hovold --- Greg, I've been aware of this overhead for a while, but never realised it was actually a regression introduced in 2009. Fixing something like this after such a long time obviously means risking a regression for anyone who is now relying on the new default behaviour instead. I still think it's reasonable in this case to restore the earlier behaviour given the penalty everyone else is paying for a minimal-latency behaviour that they likely do not need or want. Whether this should go to stable is a different question. Perhaps the stable tag is not warranted, and this should just be the default behaviour going forward? What do you think? Thanks, Johan 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 b064fad8e3ee..4bd556d9307d 100644 --- a/drivers/usb/serial/ftdi_sio.c +++ b/drivers/usb/serial/ftdi_sio.c @@ -1805,8 +1805,6 @@ 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); -- 2.10.2