[PATCH] USB: serial: cp210x: Fix error 32 when hardware flow control is enabled.

[Pho Tran]

Fix error 32 returned by CP210X_SET_MHS when hardware flow control is enabled.

The root cause of error 32 is that user application (CoolTerm, linux-serial-test)
opened cp210x device with hardware flow control then attempt to control RTS/DTR pins.
In hardware flow control, RTS/DTR pins will be controlled by hardware only,
any attempt to control those pins will cause error 32 from the device.
This fix will block MHS command(command to control RTS/DTR pins) to the device
if hardware flow control is being used.

Signed-off-by: Pho Tran <pho.tran@xxxxxxxxxx>
---
 drivers/usb/serial/cp210x.c | 27 +++++++++++++++++++++++++++
 1 file changed, 27 insertions(+)

diff --git a/drivers/usb/serial/cp210x.c b/drivers/usb/serial/cp210x.c
index fbb10dfc56e3..3694b7c62290 100644
--- a/drivers/usb/serial/cp210x.c
+++ b/drivers/usb/serial/cp210x.c
@@ -1211,6 +1211,12 @@ static int cp210x_tiocmset_port(struct usb_serial_port *port,
 		unsigned int set, unsigned int clear)
 {
 	u16 control = 0;
+	struct cp210x_flow_ctl flow_ctl;
+	u32 ctl_hs = 0;
+	u32 flow_repl = 0;
+	bool auto_dtr = false;
+	bool auto_rts = false;
+	int ret;
 
 	if (set & TIOCM_RTS) {
 		control |= CONTROL_RTS;
@@ -1231,6 +1237,27 @@ static int cp210x_tiocmset_port(struct usb_serial_port *port,
 
 	dev_dbg(&port->dev, "%s - control = 0x%.4x\n", __func__, control);
 
+	// Don't send MHS command if device in hardware flowcontrol mode
+	ret = cp210x_read_reg_block(port, CP210X_GET_FLOW, &flow_ctl, sizeof(flow_ctl));
+	if (ret)
+		return ret;
+
+	ctl_hs = le32_to_cpu(flow_ctl.ulControlHandshake);
+	flow_repl = le32_to_cpu(flow_ctl.ulFlowReplace);
+
+	if (CP210X_SERIAL_DTR_SHIFT(CP210X_SERIAL_DTR_FLOW_CTL) == (ctl_hs & CP210X_SERIAL_DTR_MASK))
+		auto_dtr = true;
+	else
+		auto_dtr = false;
+
+	if (CP210X_SERIAL_RTS_SHIFT(CP210X_SERIAL_RTS_FLOW_CTL) == (flow_repl & CP210X_SERIAL_RTS_MASK))
+		auto_rts = true;
+	else
+		auto_rts = false;
+
+	if (auto_dtr || auto_rts)
+		return 0;
+
 	return cp210x_write_u16_reg(port, CP210X_SET_MHS, control);
 }
 
-- 
2.17.1