Re: [PATCH] USB: serial: cp210x: fix CP2102N-A01 modem control

[Johan Hovold <johan@kernel.org>]

On Wed, Jun 09, 2021 at 12:00:36PM -0500, Alex Villacís Lasso wrote:
> El 9/6/21 a las 11:15, Johan Hovold escribió:
> > CP2102N revision A01 (firmware version <= 1.0.4) has a buggy
> > flow-control implementation that uses the ulXonLimit instead of
> > ulFlowReplace field of the flow-control settings structure (erratum
> > CP2102N_E104).
> >
> > A recent change that set the input software flow-control limits
> > incidentally broke RTS control for these devices when CRTSCTS is not set
> > as the new limits would always enable hardware flow control.
> >
> > Fix this by explicitly disabling flow control for the buggy firmware
> > versions and only updating the input software flow-control limits when
> > IXOFF is requested. This makes sure that the terminal settings matches
> > the default zero ulXonLimit (ulFlowReplace) for these devices.
> >
> > Reported-by: David Frey <dpfrey@gmail.com>
> > Reported-by: Alex Villacís Lasso <a_villacis@palosanto.com>
> > Fixes: f61309d9c96a ("USB: serial: cp210x: set IXOFF thresholds")
> > Cc: stable@vger.kernel.org      # 5.12
> > Signed-off-by: Johan Hovold <johan@kernel.org>
> > ---
> >   drivers/usb/serial/cp210x.c | 64 ++++++++++++++++++++++++++++++++++---
> >   1 file changed, 59 insertions(+), 5 deletions(-)
> >
> > David and Alex,
> >
> > Would you mind testing this one with your CP2108N-A01? I've verified it
> > against a CP2108N-A02 (fw 1.0.8) here.

I meant CP2102N here of course. It had been a long day...

> Applied patch and tested with ESP32 board under kernel 5.12.9:

> jun 09 11:56:00 karlalex-asus kernel: cp210x 1-9:1.0: 
> cp210x_get_fw_version - 1.0.4

> $ miniterm.py /dev/ttyUSB0 115200
> <successful connect>
> 
> jun 09 11:56:50 karlalex-asus kernel: cp210x ttyUSB0: 
> cp210x_change_speed - setting baud rate to 9600
> jun 09 11:56:50 karlalex-asus kernel: cp210x ttyUSB0: 
> cp210x_set_flow_control - ctrl = 0x00, flow = 0x00
> jun 09 11:56:50 karlalex-asus kernel: cp210x ttyUSB0: 
> cp210x_tiocmset_port - control = 0x0303
> jun 09 11:56:50 karlalex-asus kernel: cp210x ttyUSB0: 
> cp210x_change_speed - setting baud rate to 115384
> jun 09 11:56:50 karlalex-asus kernel: cp210x ttyUSB0: 
> cp210x_tiocmset_port - control = 0x0101
> jun 09 11:56:50 karlalex-asus kernel: cp210x ttyUSB0: 
> cp210x_tiocmset_port - control = 0x0202
> 
> At least in my case, this patch fixes the regression for my workflow.

Thanks for confirming. Can I add a "Tested-by" tag for you as well?

And again, thanks for the detailed report, bisection and thorough
testing throughout.

Johan