[PATCH] USB: serial: ftdi_sio: fix extreme low-latency setting

[Johan Hovold <johan@kernel.org>]

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 <a.aubert@overkiz.com>
Fixes: 557aaa7ffab6 ("ft232: support the ASYNC_LOW_LATENCY flag")
Cc: stable <stable@vger.kernel.org>	# v2.6.31
Signed-off-by: Johan Hovold <johan@kernel.org>
---

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