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>

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);