diff options
author | Johan Hovold <johan@kernel.org> | 2020-02-06 12:18:19 +0100 |
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committer | Johan Hovold <johan@kernel.org> | 2020-02-10 10:10:35 +0100 |
commit | 7c3d02285ad558691f27fde760bcd841baa27eab (patch) | |
tree | 59e031390629c18c68810914ca48e9b3b991828b | |
parent | c664a4fa8f69308b8f624cff4fa1294e9aef880d (diff) | |
download | linux-7c3d02285ad558691f27fde760bcd841baa27eab.tar.bz2 |
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>
-rw-r--r-- | drivers/usb/serial/ch341.c | 10 |
1 files changed, 10 insertions, 0 deletions
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; } |