• Johan Hovold's avatar
    USB: serial: ftdi_sio: fix extreme low-latency setting · 51cbf87a
    Johan Hovold authored
    commit c6dce262 upstream.
    
    Since commit 557aaa7f ("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 0cbd81a9 ("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: default avatarAntoine Aubert <a.aubert@overkiz.com>
    Fixes: 557aaa7f ("ft232: support the ASYNC_LOW_LATENCY flag")
    Signed-off-by: default avatarJohan Hovold <johan@kernel.org>
    Signed-off-by: default avatarBen Hutchings <ben@decadent.org.uk>
    51cbf87a
ftdi_sio.c 93 KB