Messages in this thread Patch in this message | | | From | Greg Kroah-Hartman <> | Subject | [PATCH 4.10 08/21] USB: serial: ftdi_sio: fix extreme low-latency setting | Date | Fri, 24 Feb 2017 09:40:07 +0100 |
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4.10-stable review patch. If anyone has any objections, please let me know.
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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 @@ -1802,8 +1802,6 @@ 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);
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