From 8e83622ae7ca481c76c8fd9579877f6abae64ca2 Mon Sep 17 00:00:00 2001
From: Johan Hovold <johan@kernel.org>
Date: Wed, 31 Aug 2022 10:15:24 +0200
Subject: [PATCH] USB: serial: ch341: fix lost character on LCR updates
Git-commit: 8e83622ae7ca481c76c8fd9579877f6abae64ca2
References: git-fixes
Patch-mainline: v6.0-rc4

Disable LCR updates for pre-0x30 devices which use a different (unknown)
protocol for line control and where the current register write causes
the next received character to be lost.

Note that updating LCR using the INIT command has no effect on these
devices either.

Reported-by: Jonathan Woithe <jwoithe@just42.net>
Tested-by: Jonathan Woithe <jwoithe@just42.net>
Link: https://lore.kernel.org/r/Ys1iPTfiZRWj2gXs@marvin.atrad.com.au
Fixes: 4e46c410e050 ("USB: serial: ch341: reinitialize chip on reconfiguration")
Fixes: 55fa15b5987d ("USB: serial: ch341: fix baud rate and line-control handling")
Cc: stable@vger.kernel.org      # 4.10
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Oliver Neukum <oneukum@suse.com>
---
 drivers/usb/serial/ch341.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/drivers/usb/serial/ch341.c b/drivers/usb/serial/ch341.c
index 2798fca71261..2bcce172355b 100644
--- a/drivers/usb/serial/ch341.c
+++ b/drivers/usb/serial/ch341.c
@@ -97,7 +97,10 @@ struct ch341_private {
 	u8 mcr;
 	u8 msr;
 	u8 lcr;
+
 	unsigned long quirks;
+	u8 version;
+
 	unsigned long break_end;
 };
 
@@ -265,6 +268,9 @@ static int ch341_set_baudrate_lcr(struct usb_device *dev,
 	 * (stop bits, parity and word length). Version 0x30 and above use
 	 * CH341_REG_LCR only and CH341_REG_LCR2 is always set to zero.
 	 */
+	if (priv->version < 0x30)
+		return 0;
+
 	r = ch341_control_out(dev, CH341_REQ_WRITE_REG,
 			      CH341_REG_LCR2 << 8 | CH341_REG_LCR, lcr);
 	if (r)
@@ -308,7 +314,9 @@ static int ch341_configure(struct usb_device *dev, struct ch341_private *priv)
 	r = ch341_control_in(dev, CH341_REQ_READ_VERSION, 0, 0, buffer, size);
 	if (r)
 		return r;
-	dev_dbg(&dev->dev, "Chip version: 0x%02x\n", buffer[0]);
+
+	priv->version = buffer[0];
+	dev_dbg(&dev->dev, "Chip version: 0x%02x\n", priv->version);
 
 	r = ch341_control_out(dev, CH341_REQ_SERIAL_INIT, 0, 0);
 	if (r < 0)
-- 
2.35.3