USB: serial: ch341: add basis for quirk detection

A subset of CH341 devices does not support all features, namely the
prescaler is limited to a reduced precision and there is no support for
sending a RS232 break condition. This patch adds a detection function
which will be extended to set quirk flags as they're implemented.

The author's affected device has an imprint of "340" on the
turquoise-colored plug, but not all such devices appear to be affected.

Signed-off-by: Michael Hanselmann <public@hansmi.ch>
Link: https://lore.kernel.org/r/1e1ae0da6082bb528a44ef323d4e1d3733d38858.1585697281.git.public@hansmi.ch
[ johan: use long type for quirks; rephrase and use port device for
	 messages; handle short reads; set quirk flags directly in
	 helper function ]
Cc: stable <stable@vger.kernel.org>	# 5.5
Signed-off-by: Johan Hovold <johan@kernel.org>

diff --git a/drivers/usb/serial/ch341.c b/drivers/usb/serial/ch341.c
index c5ecdcd..f2b93f4 100644 (file)
--- a/drivers/usb/serial/ch341.c
+++ b/drivers/usb/serial/ch341.c
@@ -87,6 +87,7 @@ struct ch341_private {
        u8 mcr;
        u8 msr;
        u8 lcr;
+       unsigned long quirks;
 };
 
 static void ch341_set_termios(struct tty_struct *tty,
@@ -308,6 +309,53 @@ out:       kfree(buffer);
        return r;
 }
 
+static int ch341_detect_quirks(struct usb_serial_port *port)
+{
+       struct ch341_private *priv = usb_get_serial_port_data(port);
+       struct usb_device *udev = port->serial->dev;
+       const unsigned int size = 2;
+       unsigned long quirks = 0;
+       char *buffer;
+       int r;
+
+       buffer = kmalloc(size, GFP_KERNEL);
+       if (!buffer)
+               return -ENOMEM;
+
+       /*
+        * A subset of CH34x devices does not support all features. The
+        * prescaler is limited and there is no support for sending a RS232
+        * break condition. A read failure when trying to set up the latter is
+        * used to detect these devices.
+        */
+       r = usb_control_msg(udev, usb_rcvctrlpipe(udev, 0), CH341_REQ_READ_REG,
+                           USB_TYPE_VENDOR | USB_RECIP_DEVICE | USB_DIR_IN,
+                           CH341_REG_BREAK, 0, buffer, size, DEFAULT_TIMEOUT);
+       if (r == -EPIPE) {
+               dev_dbg(&port->dev, "break control not supported\n");
+               r = 0;
+               goto out;
+       }
+
+       if (r != size) {
+               if (r >= 0)
+                       r = -EIO;
+               dev_err(&port->dev, "failed to read break control: %d\n", r);
+               goto out;
+       }
+
+       r = 0;
+out:
+       kfree(buffer);
+
+       if (quirks) {
+               dev_dbg(&port->dev, "enabling quirk flags: 0x%02lx\n", quirks);
+               priv->quirks |= quirks;
+       }
+
+       return r;
+}
+
 static int ch341_port_probe(struct usb_serial_port *port)
 {
        struct ch341_private *priv;
@@ -330,6 +378,11 @@ static int ch341_port_probe(struct usb_serial_port *port)
                goto error;
 
        usb_set_serial_port_data(port, priv);
+
+       r = ch341_detect_quirks(port);
+       if (r < 0)
+               goto error;
+
        return 0;
 
 error: kfree(priv);