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authorJohan Hovold <johan@kernel.org>2020-02-06 12:18:19 +0100
committerJohan Hovold <johan@kernel.org>2020-02-10 10:10:35 +0100
commit7c3d02285ad558691f27fde760bcd841baa27eab (patch)
tree59e031390629c18c68810914ca48e9b3b991828b /drivers
parentc664a4fa8f69308b8f624cff4fa1294e9aef880d (diff)
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>
Diffstat (limited to 'drivers')
-rw-r--r--drivers/usb/serial/ch341.c10
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;
}