usb: pl2303: increase the allowed baud rate range for the divisor based encoding method

Reinhard Max has done some tests with a PL2303HX (rev A) and a logic
analyzer and it seems, that although the PL2303HX is specified for baud
rates from 75 to 6M baud, the full divisor range can be used with the
divisor based baud rate encoding method. This corresponds to baud rates
from 46 to 24M baud.
Baud rates down to 46 baud (max. divisor) have been confirmed to work
even under heavy/permanent load, so remove the lower limit.
Baud rates up to 24M baud should really be tested carefully in "real
life" scenarios before removing the upper limit completely.
Anyway, the Windows driver allows maximum baud rates of 110% of the
specified limit, so for now, increase the upper limit to this value.
Signed-off-by: NFrank Schäfer <fschaefer.oss@googlemail.com>
Signed-off-by: NReinhard Max <max@suse.de>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/serial/pl2303.c

@@ -324,12 +324,20 @@ static int pl2303_baudrate_encode_divisor(int baud, enum pl2303_type type,
 	 */
 	unsigned int A, B;
 
-	/* Respect the specified baud rate limits */
-	baud = max_t(int, baud, 75);
+	/*
+	 * NOTE: The Windows driver allows maximum baud rates of 110% of the
+	 * specified maximium value.
+	 * Quick tests with early (2004) HX (rev. A) chips suggest, that even
+	 * higher baud rates (up to the maximum of 24M baud !) are working fine,
+	 * but that should really be tested carefully in "real life" scenarios
+	 * before removing the upper limit completely.
+	 * Baud rates smaller than the specified 75 baud are definitely working
+	 * fine.
+	 */
 	if (type == HX)
-		baud = min_t(int, baud, 6000000);
+		baud = min_t(int, baud, 6000000 * 1.1);
 	else
-		baud = min_t(int, baud, 1228800);
+		baud = min_t(int, baud, 1228800 * 1.1);
 	/* Determine factors A and B */
 	A = 0;
 	B = 12000000 * 32 / baud;  /* 12MHz */