USB: serial: ch341: fix disabled rx timer on older devices

commit 41ca302a697b64a3dab4676e01d0d11bb184737d upstream.

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>
[ johan: backport to 5.4 ]
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/usb/serial/ch341.c b/drivers/usb/serial/ch341.c
index be44d6c28df615fe0aad68f77c43fcc51f463e57..f37bde88eb5d588e0f098d617e5d6f3bdf5c0704 100644 (file)
--- a/drivers/usb/serial/ch341.c
+++ b/drivers/usb/serial/ch341.c
@@ -177,8 +177,12 @@ static int ch341_set_baudrate_lcr(struct usb_device *dev,
        /*
         * 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.
         */
-       a |= BIT(7);
+       if (priv->version > 0x27)
+               a |= BIT(7);
 
        r = ch341_control_out(dev, CH341_REQ_WRITE_REG, 0x1312, a);
        if (r)