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USB: serial: ch341: fix receiver regression

PATCH In review Johan Hovold 2 participants 3 messages Feb 6, 2020 – Feb 10, 2020

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Stable branch USB USB serial
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Johan Hovold

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#0 lore raw

PATCH 1/1 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 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 <[email protected]> # 5.5
Reported-by Jakub Nantl <[email protected]>
Tested-by Jakub Nantl <[email protected]>
Signed-off-by Johan Hovold <[email protected]>

---

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(+)
drivers/usb/serial/ch341.c +10 -0
index df582fe855f0..415c3d31492b 100644
@@ -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;
 }
Signature · 1 line
2.24.1
maintainer Reviewed-by on v1 1/1
#1 lore raw
Quoted 12 lines · Johan Hovold

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.

Quoted 5 lines
Fixes 35714565089e ("USB: serial: ch341: reimplement line-speed handling")
Cc stable <[email protected]> # 5.5
Reported-by Jakub Nantl <[email protected]>
Tested-by Jakub Nantl <[email protected]>
Signed-off-by Johan Hovold <[email protected]>
Reviewed-by Greg Kroah-Hartman <[email protected]>
author maintainer
#2 lore raw
Quoted 13 lines · Greg Kroah-Hartman
Quoted 12 lines · Johan Hovold

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.

Quoted 7 lines
Quoted 5 lines
Fixes 35714565089e ("USB: serial: ch341: reimplement line-speed handling")
Cc stable <[email protected]> # 5.5
Reported-by Jakub Nantl <[email protected]>
Tested-by Jakub Nantl <[email protected]>
Signed-off-by Johan Hovold <[email protected]>
Reviewed-by Greg Kroah-Hartman <[email protected]>

Thanks for reviewing. Now applied.

Johan