linux-kernel - [PATCH] Proper RTS control when buffers fill

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Date:   Wed, 18 Dec 2019 20:40:48 +0000
From:   Brant Merryman <Brant.Merryman@...abs.com>
To:     Johan Hovold <johan@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
CC:     Brant Merryman <Brant.Merryman@...abs.com>,
        Richard Hendricks <Richard.Hendricks@...abs.com>
Subject: [PATCH] Proper RTS control when buffers fill

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: Brant Merryman <brant.merryman@...abs.com>
---
 drivers/usb/serial/cp210x.c | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/drivers/usb/serial/cp210x.c b/drivers/usb/serial/cp210x.c
index bcceb4ad8be0..74c9f3822bd2 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);
 
@@ -1015,8 +1016,24 @@ static void cp210x_get_termios_port(struct usb_serial_port *port,
 	cp210x_read_reg_block(port, CP210X_GET_FLOW, &flow_ctl,
 			sizeof(flow_ctl));
 	ctl_hs = le32_to_cpu(flow_ctl.ulControlHandshake);
+	flow_repl = le32_to_cpu(flow_ctl.ulFlowReplace);
 	if (ctl_hs & CP210X_SERIAL_CTS_HANDSHAKE) {
 		dev_dbg(dev, "%s - flow control = CRTSCTS\n", __func__);
+		/*
+		 * 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.
+		 */
+		flow_repl &= ~CP210X_SERIAL_RTS_MASK;
+		flow_repl |= CP210X_SERIAL_RTS_SHIFT(CP210X_SERIAL_RTS_FLOW_CTL);
+
+		flow_ctl.ulControlHandshake = cpu_to_le32(ctl_hs);
+		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__);
--