patch "USB: pl2303: fix baud-rate divisor calculations" added to usb-next

patch "USB: pl2303: fix baud-rate divisor calculations" added to usb-next

[gregkh]

This is a note to let you know that I've just added the patch titled

    USB: pl2303: fix baud-rate divisor calculations

to my usb git tree which can be found at
    git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/usb.git
in the usb-next branch.

The patch will show up in the next release of the linux-next tree
(usually sometime within the next 24 hours during the week.)

The patch will also be merged in the next major kernel release
during the merge window.

If you have any questions about this process, please let me know.


>From 49bda21266fdf195142e8b5dea057f09e96ada9f Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Micha=C5=82=20Pecio?= <michal.pecio@gmail.com>
Date: Sun, 26 Jul 2015 11:14:34 +0200
Subject: USB: pl2303: fix baud-rate divisor calculations

This commit fixes the following issues:

1. The 9th bit of buf was believed to be the LSB of divisor's
exponent, but the hardware interprets it as MSB (9th bit) of the
mantissa. The exponent is actually one bit shorter and applies
to base 4, not 2 as previously believed.

2. Loop iterations doubled the exponent instead of incrementing.

3. The exponent wasn't checked for overflow.

4. The function returned requested rate instead of actual rate.

Due to issue #2, the old code deviated from the wrong formula
described in #1 and actually yielded correct rates when divisor
was lower than 4096 by using exponents of 0, 2 or 4 base-2,
interpreted as 0, 1, 2 base-4 with the 9th mantissa bit clear.
However, at 93.75 kbaud or less the rate turned out too slow
due to #2 or too fast due to #2 and #3.

I tested this patch by sending and validating 0x00,0x01,..,0xff
to an FTDI dongle at 234, 987, 2401, 9601, 31415, 115199, 250k,
500k, 750k, 1M, 1.5M, 3M+1 baud. All rates passed.

I also used pv to check speed at some rates unsupported by FTDI:
45 (the lowest possible), 2M, 4M, 5M and 6M-1. Looked sane.

Signed-off-by: Michal Pecio <michal.pecio@gmail.com>
Fixes: 399aa9a75ad3 ("USB: pl2303: use divisors for unsupported baud
rates")
Cc: stable <stable@vger.kernel.org> # v3.18
[johan: update summary ]
Signed-off-by: Johan Hovold <johan@kernel.org>
---
 drivers/usb/serial/pl2303.c | 35 ++++++++++++++++++++++++++---------
 1 file changed, 26 insertions(+), 9 deletions(-)

diff --git a/drivers/usb/serial/pl2303.c b/drivers/usb/serial/pl2303.c
index f5257af33ecf..ae682e4eeaef 100644
--- a/drivers/usb/serial/pl2303.c
+++ b/drivers/usb/serial/pl2303.c
@@ -362,21 +362,38 @@ static speed_t pl2303_encode_baud_rate_direct(unsigned char buf[4],
 static speed_t pl2303_encode_baud_rate_divisor(unsigned char buf[4],
 								speed_t baud)
 {
-	unsigned int tmp;
+	unsigned int baseline, mantissa, exponent;
 
 	/*
 	 * Apparently the formula is:
-	 * baudrate = 12M * 32 / (2^buf[1]) / buf[0]
+	 *   baudrate = 12M * 32 / (mantissa * 4^exponent)
+	 * where
+	 *   mantissa = buf[8:0]
+	 *   exponent = buf[11:9]
 	 */
-	tmp = 12000000 * 32 / baud;
+	baseline = 12000000 * 32;
+	mantissa = baseline / baud;
+	if (mantissa == 0)
+		mantissa = 1;	/* Avoid dividing by zero if baud > 32*12M. */
+	exponent = 0;
+	while (mantissa >= 512) {
+		if (exponent < 7) {
+			mantissa >>= 2;	/* divide by 4 */
+			exponent++;
+		} else {
+			/* Exponent is maxed. Trim mantissa and leave. */
+			mantissa = 511;
+			break;
+		}
+	}
+
 	buf[3] = 0x80;
 	buf[2] = 0;
-	buf[1] = (tmp >= 256);
-	while (tmp >= 256) {
-		tmp >>= 2;
-		buf[1] <<= 1;
-	}
-	buf[0] = tmp;
+	buf[1] = exponent << 1 | mantissa >> 8;
+	buf[0] = mantissa & 0xff;
+
+	/* Calculate and return the exact baud rate. */
+	baud = (baseline / mantissa) >> (exponent << 1);
 
 	return baud;
 }
-- 
2.5.0