From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-lf0-f68.google.com ([209.85.215.68]:34306 "EHLO mail-lf0-f68.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751404AbdAYQCe (ORCPT ); Wed, 25 Jan 2017 11:02:34 -0500 Date: Wed, 25 Jan 2017 17:02:33 +0100 From: Johan Hovold To: Greg Kroah-Hartman Cc: Johan Hovold , linux-usb@vger.kernel.org, Antoine Aubert , stable@vger.kernel.org Subject: Re: [PATCH] USB: serial: ftdi_sio: fix extreme low-latency setting Message-ID: <20170125160233.GE20261@localhost> References: <20170125143520.10982-1-johan@kernel.org> <20170125150647.GB16839@kroah.com> <20170125151436.GD20261@localhost> <20170125154201.GA21106@kroah.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170125154201.GA21106@kroah.com> Sender: stable-owner@vger.kernel.org List-ID: On Wed, Jan 25, 2017 at 04:42:01PM +0100, Greg Kroah-Hartman wrote: > On Wed, Jan 25, 2017 at 04:14:36PM +0100, Johan Hovold wrote: > > On Wed, Jan 25, 2017 at 04:06:47PM +0100, Greg Kroah-Hartman wrote: > > > On Wed, Jan 25, 2017 at 03:35:20PM +0100, Johan Hovold wrote: > > > > 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? > > > > > > I think the stable tag is warrented here. Do you want me to take this > > > patch now into my usb-linus tree, or will you include it in a pull > > > request? > > > > I'll include it in a pull request. I was gonna apply it for -next, but > > if you prefer I can send it along with some new device ids I have queued > > up for 4.10-rc? > > -next is probably good, it's not like there is a rush for it :) True. Don't really wanna think about how much power has been wasted on these insane interrupt rates considering the number of these devices that are out there though... Now applied for -next, with a stable dependency on patch already in my tree which did not need to be backported while ASYNC_LOW_LATENCY was set. Thanks, Johan