[Regression] CH341 USB-serial converter passes data in 32 byte chunks

Jonathan Woithe
2022-07-12T12:21:22Z

Hi all

For many years I have been using a CH341 USB-serial converter (VID:PID
4348:5523) to drive a microcontroller programming dongle. This was under
kernel 4.4.14. When I recently upgraded the machine to a 5.15.27 kernel the
programmer stopped working, reporting timeouts. Using a loopback cable and
minicom I discovered that under 5.15.27 data was only delivered back to
minicom in blocks of 32 characters. This explains why the programming
software reported timeouts. It seems that either outgoing data is blocked
until 32 bytes are ready for transmission, or receive data is only reported
in blocks of 32 bytes.

Under 4.4.14 (and all kernels prior to 4.10.0), individual characters are
echoed back by minicom as soon as they hare pressed on the keyboard.

As far as I can tell, the regression affects all kernels since 4.10.0.

I have done a git bisect which identified the following commit as the source
of the problem.

commit 55fa15b5987db22b4f35d3f0798928c126be5f1c
Author: Johan Hovold <johan@kernel.org>
Date: Fri Jan 6 19:15:16 2017 +0100

USB: serial: ch341: fix baud rate and line-control handling

Revert to using direct register writes to set the divisor and
line-control registers.

A recent change switched to using the init vendor command to update
these registers, something which also enabled support for CH341A
devices. It turns out that simply setting bit 7 in the divisor register
is sufficient to support CH341A and specifically prevent data from being
buffered until a full endpoint-size packet (32 bytes) has been received.

Using the init command also had the side-effect of temporarily
deasserting the DTR/RTS signals on every termios change (including
initialisation on open) something which for example could cause problems
in setups where DTR is used to trigger a reset.

Fixes: 4e46c410e050 ("USB: serial: ch341: reinitialize chip on
reconfiguration")
Signed-off-by: Johan Hovold <johan@kernel.org>

It would be great if this regression could be addressed. At present I must
boot a pre-4.10 kernel whenever I need to use the programming dongle with
this converter.

Please let me know if there is anything I can do to help resolve the
problem.

Regards
jonathan