USB: pl2303: distinguish between original and cloned HX chips

According to Prolific, several (unauthorized) cheap and less functional
clones of the PL2303HX chip are in circulation. [1]
I've had the chance to test such a cloned device and it turned out that
it doesn't support any baud rates above 115200 baud (original: 6 Mbaud)
It also doesn't support the divisior based baud rate encoding method,
so no continuous baud rate adjustment is possible.
Nevertheless, these devices have been working (unintentionally) with
the driver up to commit 61fa8d694b ("pl2303: also use the divisor based
baud rate encoding method for baud rates < 115200 with HX chips"), and
this commit broke support for them.
Fortunately, it is pretty simple to distinguish between the original
and the cloned HX chips, so I've added a check and an extra chip type
to keep the clones working.
The same check is used by the latest Prolific Windows driver, so it
should be solid.

[1] http://www.prolific.com.tw/US/ShowProduct.aspx?p_id=225&pcid=41

Signed-off-by: Frank Schäfer <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/usb/serial/pl2303.c

···
 	HX_TA,		/* HX(A) / X(A) / TA version  */ /* TODO: improve */
 	HXD_EA_RA_SA,	/* HXD / EA / RA / SA version */ /* TODO: improve */
 	TB,		/* TB version */
+	HX_CLONE,	/* Cheap and less functional clone of the HX chip */
 };
 /*
  * NOTE: don't know the difference between type 0 and type 1,
···
 		 * the device descriptors of the X/HX, HXD, EA, RA, SA, TA, TB
 		 */
 		if (le16_to_cpu(serial->dev->descriptor.bcdDevice) == 0x300) {
-			type = HX_TA;
-			type_str = "X/HX/TA";
+			/* Check if the device is a clone */
+			pl2303_vendor_read(0x9494, 0, serial, buf);
+			/*
+			 * NOTE: Not sure if this read is really needed.
+			 * The HX returns 0x00, the clone 0x02, but the Windows
+			 * driver seems to ignore the value and continues.
+			 */
+			pl2303_vendor_write(0x0606, 0xaa, serial);
+			pl2303_vendor_read(0x8686, 0, serial, buf);
+			if (buf[0] != 0xaa) {
+				type = HX_CLONE;
+				type_str = "X/HX clone (limited functionality)";
+			} else {
+				type = HX_TA;
+				type_str = "X/HX/TA";
+			}
+			pl2303_vendor_write(0x0606, 0x00, serial);
 		} else if (le16_to_cpu(serial->dev->descriptor.bcdDevice)
 								     == 0x400) {
 			type = HXD_EA_RA_SA;
···
 {
 	/*
 	 * NOTE: Only the values defined in baud_sup are supported !
-	 *       => if unsupported values are set, the PL2303 seems to
-	 *	    use 9600 baud (at least my PL2303X always does)
+	 * => if unsupported values are set, the PL2303 uses 9600 baud instead
+	 * => HX clones just don't work at unsupported baud rates < 115200 baud,
+	 *    for baud rates > 115200 they run at 115200 baud
 	 */
 	const int baud_sup[] = { 75, 150, 300, 600, 1200, 1800, 2400, 3600,
 				 4800, 7200, 9600, 14400, 19200, 28800, 38400,
···
 	 * NOTE: With the exception of type_0/1 devices, the following
 	 * additional baud rates are supported (tested with HX rev. 3A only):
 	 * 110*, 56000*, 128000, 134400, 161280, 201600, 256000*, 268800,
-	 * 403200, 806400.	(*: not HX)
+	 * 403200, 806400.	(*: not HX and HX clones)
 	 *
 	 * Maximum values: HXD, TB: 12000000; HX, TA: 6000000;
-	 *                 type_0+1: 1228800; RA: 921600; SA: 115200
+	 *                 type_0+1: 1228800; RA: 921600; HX clones, SA: 115200
 	 *
 	 * As long as we are not using this encoding method for anything else
-	 * than the type_0+1 and HX chips, there is no point in complicating
-	 * the code to support them.
+	 * than the type_0+1, HX and HX clone chips, there is no point in
+	 * complicating the code to support them.
 	 */
 	int i;
 
···
 		baud = min_t(int, baud, 6000000);
 	else if (type == type_0 || type == type_1)
 		baud = min_t(int, baud, 1228800);
+	else if (type == HX_CLONE)
+		baud = min_t(int, baud, 115200);
 	/* Direct (standard) baud rate encoding method */
 	put_unaligned_le32(baud, buf);
 
···
 	/*
 	 * Divisor based baud rate encoding method
 	 *
-	 * NOTE: it's not clear if the type_0/1 chips support this method
+	 * NOTE: HX clones do NOT support this method.
+	 * It's not clear if the type_0/1 chips support it.
 	 *
 	 * divisor = 12MHz * 32 / baudrate = 2^A * B
 	 *
···
 	 * 1) Direct method: encodes the baud rate value directly
 	 *    => supported by all chip types
 	 * 2) Divisor based method: encodes a divisor to a base value (12MHz*32)
-	 *    => supported by HX chips (and likely not by type_0/1 chips)
+	 *    => not supported by HX clones (and likely type_0/1 chips)
 	 *
 	 * NOTE: Although the divisor based baud rate encoding method is much
 	 * more flexible, some of the standard baud rate values can not be
···
 	 * the device likely uses the same baud rate generator for both methods
 	 * so that there is likley no difference.
 	 */
-	if (type == type_0 || type == type_1)
+	if (type == type_0 || type == type_1 || type == HX_CLONE)
 		baud = pl2303_baudrate_encode_direct(baud, type, buf);
 	else
 		baud = pl2303_baudrate_encode_divisor(baud, type, buf);
···
 	result = usb_control_msg(serial->dev, usb_sndctrlpipe(serial->dev, 0),
 				 BREAK_REQUEST, BREAK_REQUEST_TYPE, state,
 				 0, NULL, 0, 100);
+	/* NOTE: HX clones don't support sending breaks, -EPIPE is returned */
 	if (result)
 		dev_err(&port->dev, "error sending break = %d\n", result);
 }