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ASYNC_LOW_LATENCY is set by userspace via TIOCSSERIAL, used by setserial(8), libftdi and certain tcsetattr paths. The interaction with the existing sysfs latency_timer attribute is surprising: once any of those tools has set the flag, a later write of "16" (or any other value) to /sys/bus/usb-serial/devices/ttyUSBx/latency_timer is silently clamped to 1 and never reaches the chip. The store path is the most explicit way userspace can ask for a particular latency_timer value; treat it as authoritative. On an explicit sysfs write, clear ASYNC_LOW_LATENCY before calling write_latency_timer() so the requested value is what the chip register actually receives. The override is logged at debug level (dev_dbg) for diagnostics only. Reads continue to honour ASYNC_LOW_LATENCY (returning "1") so any userspace that previously inspected the attribute to confirm low-latency mode keeps working until it does its own explicit write. Signed-off-by: Chinna Mopurigari Naveen Kumar Reddy --- drivers/usb/serial/ftdi_sio.c | 17 +++++++++++++++++ 1 file changed, 17 insertions(+) diff --git a/drivers/usb/serial/ftdi_sio.c b/drivers/usb/serial/ftdi_sio.c index 7aaa7fc1be71..d2547ad76423 100644 --- a/drivers/usb/serial/ftdi_sio.c +++ b/drivers/usb/serial/ftdi_sio.c @@ -1683,6 +1683,23 @@ static ssize_t latency_timer_store(struct device *dev, if (kstrtou8(valbuf, 10, &v)) return -EINVAL; + /* + * An explicit sysfs write wins over the legacy ASYNC_LOW_LATENCY + * tty-flag override. Without this, if any userspace tool + * (setserial(8), libftdi, certain tcsetattr paths) had set + * ASYNC_LOW_LATENCY via TIOCSSERIAL, write_latency_timer() would + * silently clamp the chip register to 1 regardless of what was + * written to sysfs. Clearing the flag here makes sysfs the + * authoritative source so the next chip-side write uses exactly + * the value the caller asked for. + */ + if (priv->flags & ASYNC_LOW_LATENCY) { + dev_dbg(&port->dev, + "explicit latency_timer=%u clears ASYNC_LOW_LATENCY flag\n", + v); + priv->flags &= ~ASYNC_LOW_LATENCY; + } + priv->latency = v; rv = write_latency_timer(port); if (rv < 0) -- 2.43.0