[PATCH v3 2/2] USB: serial: cp210x: Proper RTS control when buffers fill

[Phu Luu <Phu.Luu@silabs.com>]

From: Brant Merryman <brant.merryman@silabs.com>

CP210x hardware disables auto-RTS but leaves auto-CTS when
in hardware flow control mode and UART on cp210x hardware
is disabled. This allows data to flow out, but new data
will not come into the port. When re-opening the port, if
auto-CTS is enabled on the cp210x, then auto-RTS must be
re-enabled in the driver.

Signed-off-by: Phu Luu <phu.luu@silabs.com>
Signed-off-by: Brant Merryman <brant.merryman@silabs.com>
---
06/09/2020: Patch v3 2/2 Modified based on feedback from Johan Hovold <johan@kernel.org>
12/18/2019: Patch v2 Broken into two patches and modified based on feedback from Johan Hovold <johan@kernel.org>
12/09/2019: Initial submission of patch "Proper RTS control when buffers fill"

 drivers/usb/serial/cp210x.c | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/drivers/usb/serial/cp210x.c b/drivers/usb/serial/cp210x.c
index bcceb4ad8be0..091441b1c5b9 100644
--- a/drivers/usb/serial/cp210x.c
+++ b/drivers/usb/serial/cp210x.c
@@ -917,6 +917,7 @@ static void cp210x_get_termios_port(struct usb_serial_port *port,
 	u32 baud;
 	u16 bits;
 	u32 ctl_hs;
+	u32 flow_repl;
 
 	cp210x_read_u32_reg(port, CP210X_GET_BAUDRATE, &baud);
 
@@ -1017,6 +1018,23 @@ static void cp210x_get_termios_port(struct usb_serial_port *port,
 	ctl_hs = le32_to_cpu(flow_ctl.ulControlHandshake);
 	if (ctl_hs & CP210X_SERIAL_CTS_HANDSHAKE) {
 		dev_dbg(dev, "%s - flow control = CRTSCTS\n", __func__);
+		/*
+		 * When the port is closed, the CP210x hardware disables
+		 * auto-RTS and RTS is deasserted but it leaves auto-CTS when
+		 * in hardware flow control mode. This prevents new data from
+		 * being received by the port. When re-opening the port, if
+		 * auto-CTS is enabled on the cp210x, then auto-RTS must be
+		 * re-enabled in the driver.
+		 */
+		flow_repl = le32_to_cpu(flow_ctl.ulFlowReplace);
+		flow_repl &= ~CP210X_SERIAL_RTS_MASK;
+		flow_repl |= CP210X_SERIAL_RTS_SHIFT(CP210X_SERIAL_RTS_FLOW_CTL);
+		flow_ctl.ulFlowReplace = cpu_to_le32(flow_repl);
+		cp210x_write_reg_block(port,
+				CP210X_SET_FLOW,
+				&flow_ctl,
+				sizeof(flow_ctl));
+
 		cflag |= CRTSCTS;
 	} else {
 		dev_dbg(dev, "%s - flow control = NONE\n", __func__);
-- 
2.17.0