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@kernel.org 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@vger.kernel.org # 5.5 Reported-by: Jakub Nantl jn@forever.cz Tested-by: Jakub Nantl jn@forever.cz Reviewed-by: Greg Kroah-Hartman gregkh@linuxfoundation.org Signed-off-by: Johan Hovold johan@kernel.org --- 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; }