* [PATCH] USB: serial: ch341: fix receiver regression
@ 2020-02-06 11:18 Johan Hovold
2020-02-06 11:45 ` Greg KH
0 siblings, 1 reply; 3+ messages in thread
From: Johan Hovold @ 2020-02-06 11:18 UTC (permalink / raw)
To: Johan Hovold
Cc: Jakub Nantl, Jonathan Olds, Michael Dreher, linux-usb, stable
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 is close to nominal.
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>
Signed-off-by: Johan Hovold <johan@kernel.org>
---
I was able to reproduce this using a MAX232 tranceiver at 115200 even
with a close-to-nominal input rate, so the receiver is definitely more
sensitive to errors with the 2-prescaler disabled.
Note that I had this step in the first version of the new algorithm, but
couldn't convince myself that it wasn't redundant.
Reverse-engineering is fun.
Johan
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 df582fe855f0..415c3d31492b 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;
}
--
2.24.1
^ permalink raw reply related [flat|nested] 3+ messages in thread
* Re: [PATCH] USB: serial: ch341: fix receiver regression
2020-02-06 11:18 [PATCH] USB: serial: ch341: fix receiver regression Johan Hovold
@ 2020-02-06 11:45 ` Greg KH
2020-02-10 9:35 ` Johan Hovold
0 siblings, 1 reply; 3+ messages in thread
From: Greg KH @ 2020-02-06 11:45 UTC (permalink / raw)
To: Johan Hovold
Cc: Jakub Nantl, Jonathan Olds, Michael Dreher, linux-usb, stable
On Thu, Feb 06, 2020 at 12:18:19PM +0100, Johan Hovold wrote:
> 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 is close to nominal.
>
> 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>
> Signed-off-by: Johan Hovold <johan@kernel.org>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: [PATCH] USB: serial: ch341: fix receiver regression
2020-02-06 11:45 ` Greg KH
@ 2020-02-10 9:35 ` Johan Hovold
0 siblings, 0 replies; 3+ messages in thread
From: Johan Hovold @ 2020-02-10 9:35 UTC (permalink / raw)
To: Greg KH
Cc: Johan Hovold, Jakub Nantl, Jonathan Olds, Michael Dreher,
linux-usb, stable
On Thu, Feb 06, 2020 at 11:45:46AM +0000, Greg Kroah-Hartman wrote:
> On Thu, Feb 06, 2020 at 12:18:19PM +0100, Johan Hovold wrote:
> > 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 is close to nominal.
> >
> > 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>
> > Signed-off-by: Johan Hovold <johan@kernel.org>
>
> Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Thanks for reviewing. Now applied.
Johan
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