[PATCH 4.9 19/32] USB: serial: ftdi_sio: fix extreme low-latency setting

From	Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Newsgroups	linux.kernel
Subject	[PATCH 4.9 19/32] USB: serial: ftdi_sio: fix extreme low-latency setting
Date	2017-02-24 10:00 +0100
Message-ID	<tejTl-66Y-49@gated-at.bofh.it> (permalink)
References	<tejzY-5Xe-19@gated-at.bofh.it>
Organization	linux.* mail to news gateway

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4.9-stable review patch.  If anyone has any objections, please let me know.

------------------

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);

Thread

[PATCH 4.9 00/32] 4.9.13-stable review Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-02-24 09:40 +0100
  [PATCH 4.9 27/32] goldfish: Sanitize the broken interrupt handler Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-02-24 09:50 +0100
  [PATCH 4.9 23/32] USB: serial: opticon: fix CTS retrieval at open Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-02-24 09:50 +0100
  [PATCH 4.9 30/32] block: fix double-free in the failure path of cgwb_bdi_init() Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-02-24 09:50 +0100
  [PATCH 4.9 13/32] ip: fix IP_CHECKSUM handling Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-02-24 09:50 +0100
  [PATCH 4.9 28/32] netfilter: nf_ct_helper: warn when not applying default helper assignment Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-02-24 09:50 +0100
  [PATCH 4.9 31/32] rtlwifi: rtl_usb: Fix for URB leaking when doing ifconfig up/down Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-02-24 09:50 +0100
  [PATCH 4.9 17/32] USB: serial: cp210x: add new IDs for GE Bx50v3 boards Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-02-24 09:50 +0100
  [PATCH 4.9 16/32] USB: serial: mos7840: fix another NULL-deref at open Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-02-24 09:50 +0100
  [PATCH 4.9 25/32] USB: serial: console: fix uninitialised spinlock Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-02-24 09:50 +0100
  [PATCH 4.9 11/32] irda: Fix lockdep annotations in hashbin_delete(). Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-02-24 09:50 +0100
  [PATCH 4.9 24/32] USB: serial: ark3116: fix register-accessor error handling Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-02-24 09:50 +0100
  [PATCH 4.9 15/32] tty: serial: msm: Fix module autoload Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-02-24 09:50 +0100
  [PATCH 4.9 29/32] ACPICA: Linuxize: Restore and fix Intel compiler build Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-02-24 09:50 +0100
  [PATCH 4.9 32/32] xfs: clear delalloc and cache on buffered write failure Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-02-24 09:50 +0100
  [PATCH 4.9 22/32] USB: serial: spcp8x5: fix modem-status handling Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-02-24 09:50 +0100
  [PATCH 4.9 08/32] net: neigh: Fix netevent NETEVENT_DELAY_PROBE_TIME_UPDATE notification Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-02-24 09:50 +0100
  [PATCH 4.9 26/32] x86/platform/goldfish: Prevent unconditional loading Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-02-24 09:50 +0100
  [PATCH 4.9 12/32] ptr_ring: fix race conditions when resizing Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-02-24 10:00 +0100
  [PATCH 4.9 02/32] kcm: fix a null pointer dereference in kcm_sendmsg() Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-02-24 10:00 +0100
  [PATCH 4.9 07/32] packet: Do not call fanout_release from atomic contexts Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-02-24 10:00 +0100
  [PATCH 4.9 04/32] net/llc: avoid BUG_ON() in skb_orphan() Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-02-24 10:00 +0100
  [PATCH 4.9 14/32] net: socket: fix recvmmsg not returning error from sock_error Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-02-24 10:00 +0100
  [PATCH 4.9 10/32] vxlan: fix oops in dev_fill_metadata_dst Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-02-24 10:00 +0100
  [PATCH 4.9 03/32] net/mlx5e: Disable preemption when doing TC statistics upcall Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-02-24 10:00 +0100
  [PATCH 4.9 19/32] USB: serial: ftdi_sio: fix extreme low-latency setting Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-02-24 10:00 +0100
  Re: [PATCH 4.9 00/32] 4.9.13-stable review Guenter Roeck <linux@roeck-us.net> - 2017-02-24 18:30 +0100
  Re: [PATCH 4.9 00/32] 4.9.13-stable review Shuah Khan <shuahkh@osg.samsung.com> - 2017-02-24 19:20 +0100
  Re: [PATCH 4.9 00/32] 4.9.13-stable review Alexandre Belloni <alexandre.belloni@free-electrons.com> - 2017-02-27 00:00 +0100

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