Patch "USB: serial: ftdi_sio: fix extreme low-latency setting" has been added to the 4.4-stable tree

[<gregkh@linuxfoundation.org>]

This is a note to let you know that I've just added the patch titled

    USB: serial: ftdi_sio: fix extreme low-latency setting

to the 4.4-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     usb-serial-ftdi_sio-fix-extreme-low-latency-setting.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@vger.kernel.org> know about it.


>From c6dce2626606ef16434802989466636bc28c1419 Mon Sep 17 00:00:00 2001
From: Johan Hovold <johan@kernel.org>
Date: Wed, 25 Jan 2017 15:35:20 +0100
Subject: USB: serial: ftdi_sio: fix extreme low-latency setting

From: Johan Hovold <johan@kernel.org>

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 <a.aubert@overkiz.com>
Fixes: 557aaa7ffab6 ("ft232: support the ASYNC_LOW_LATENCY flag")
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 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
@@ -1807,8 +1807,6 @@ 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);
 


Patches currently in stable-queue which might be from johan@kernel.org are

queue-4.4/usb-serial-ftdi_sio-fix-modem-status-error-handling.patch
queue-4.4/usb-serial-cp210x-add-new-ids-for-ge-bx50v3-boards.patch
queue-4.4/usb-serial-ark3116-fix-register-accessor-error-handling.patch
queue-4.4/usb-serial-digi_acceleport-fix-oob-data-sanity-check.patch
queue-4.4/usb-serial-ftdi_sio-fix-line-status-over-reporting.patch
queue-4.4/usb-serial-ftdi_sio-fix-extreme-low-latency-setting.patch
queue-4.4/usb-serial-spcp8x5-fix-modem-status-handling.patch
queue-4.4/usb-serial-opticon-fix-cts-retrieval-at-open.patch
queue-4.4/usb-serial-mos7840-fix-another-null-deref-at-open.patch