patch "USB: serial: ch341: fix receiver regression" added to usb-linus

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

USB: serial: ch341: fix receiver regression

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-linus 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 hopefully also be merged in Linus's tree for the next -rc kernel release.

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

From 7c3d02285ad558691f27fde760bcd841baa27eab Mon Sep 17 00:00:00 2001
From: Johan Hovold <[email protected]>
Date: Thu, 6 Feb 2020 12:18:19 +0100
Subject: USB: serial: ch341: fix receiver regression

While assumed not to make a difference, not using the factor-2 prescaler makes the receiver more susceptible to errors.

Specifically, there have been reports of problems with devices that cannot generate a 115200 rate with a smaller error than 2.1% (e.g. 117647 bps). But this can also be reproduced with a low-speed RS232 tranceiver at 115200 when the input rate matches the nominal rate.

So whenever possible, enable the factor-2 prescaler and halve the divisor in order to use settings closer to that of the previous algorithm.

---
 drivers/usb/serial/ch341.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

index d3f420f3a083..c5ecdcd51ffc 100644
@@ -205,6 +205,16 @@ static int ch341_get_divisor(speed_t speed)
 			16 * speed - 16 * CH341_CLKRATE / (clk_div * (div + 1)))
 		div++;
 
+	/*
+	 * Prefer lower base clock (fact = 0) if even divisor.
+	 *
+	 * Note that this makes the receiver more tolerant to errors.
+	 */
+	if (fact == 1 && div % 2 == 0) {
+		div /= 2;
+		fact = 0;
+	}
+
 	return (0x100 - div) << 8 | fact << 2 | ps;
 }
 

2.25.0