[PATCH v3 2/2] USB: serial: ftdi_sio: make explicit latency_timer sysfs write authoritative

[Chinna Mopurigari Naveen Kumar Reddy]

write_latency_timer() clamps the value programmed into the FT chip's
per-channel latency_timer register to 1 whenever ASYNC_LOW_LATENCY is
set in priv->flags.  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 <naveen.reddy@xxxxxxxxxxxx>
---
 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