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 <johan@xxxxxxxxxx>
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.
Fixes: 35714565089e ("USB: serial: ch341: reimplement line-speed handling")
Cc: stable <stable@xxxxxxxxxxxxxxx> # 5.5
Reported-by: Jakub Nantl <jn@xxxxxxxxxx>
Tested-by: Jakub Nantl <jn@xxxxxxxxxx>
Reviewed-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
Signed-off-by: Johan Hovold <johan@xxxxxxxxxx>
---
drivers/usb/serial/ch341.c | 10 ++++++++++
1 file changed, 10 insertions(+)
diff --git a/drivers/usb/serial/ch341.c b/drivers/usb/serial/ch341.c
index d3f420f3a083..c5ecdcd51ffc 100644
--- a/drivers/usb/serial/ch341.c
+++ b/drivers/usb/serial/ch341.c
@@ -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
