From 41ca302a697b64a3dab4676e01d0d11bb184737d Mon Sep 17 00:00:00 2001
From: Johan Hovold <johan@kernel.org>
Date: Wed, 31 Aug 2022 10:15:25 +0200
Subject: [PATCH] USB: serial: ch341: fix disabled rx timer on older devices
Git-commit: 41ca302a697b64a3dab4676e01d0d11bb184737d
References: git-fixes
Patch-mainline: v6.0-rc4

At least one older CH341 appears to have the RX timer enable bit
inverted so that setting it disables the RX timer and prevents the FIFO
from emptying until it is full.

Only set the RX timer enable bit for devices with version newer than
0x27 (even though this probably affects all pre-0x30 devices).

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")
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(-)

--- a/drivers/usb/serial/ch341.c
+++ b/drivers/usb/serial/ch341.c
@@ -255,7 +255,15 @@ static int ch341_set_baudrate_lcr(struct
 	if (priv->version < 0x30)
 		return 0;
 
-	val |= BIT(7);
+	/*
+	 * CH341A buffers data until a full endpoint-size packet (32 bytes)
+	 * has been received unless bit 7 is set.
+	 *
+	 * At least one device with version 0x27 appears to have this bit
+	 * inverted.
+	 */
+	if (priv->version > 0x27)
+		val |= BIT(7);
 
 	r = ch341_control_out(dev, CH341_REQ_WRITE_REG, 0x1312, val);
 	if (r)