Re: non-standard baud rates with Prolific 2303 USB-serial

[Johan Hovold <johan@kernel.org>]

On Tue, Feb 23, 2021 at 09:58:47AM -0500, Michael G. Katzmann wrote:

> Well I found I have another Prolific device here that I got from
> Adafruit (https://www.adafruit.com/product/954)
> 
> It also has a Prolific 2303 and behaves exactly like the other one.
> i.e. with the current Linux driver it gives a very high baudrate when
> set to 110 bd and gives 110bd when the modifications are made to the
> driver that I found.

Interesting.

> Is it that we are presuming that what Prolific is telling us is true
> and only Joe and I are actually measuring the data rate?  (i.e. why
> does the Prolific Windows driver set the values as Joe found ???)

I'm starting to think they've added some alternate baud rate encoding in
order to make life harder for the people pushing (or unknowingly buying)
counterfeit devices.

As you say, why else would the Windows driver support this encoding?

The driver might be triggering on the device descriptors or some control
request so having a packet trace might provide some insight.

I think Joe may already have collected one?
 
> Adafruit device:
> 
> Bus 001 Device 013: ID 067b:2303 Prolific Technology, Inc. PL2303 Serial Port
> Device Descriptor:
>   bLength                18
>   bDescriptorType         1
>   bcdUSB               2.00
>   bDeviceClass            0
>   bDeviceSubClass         0
>   bDeviceProtocol         0
>   bMaxPacketSize0        64
>   idVendor           0x067b Prolific Technology, Inc.
>   idProduct          0x2303 PL2303 Serial Port
>   bcdDevice            3.00
>   iManufacturer           1 Prolific Technology Inc.
>   iProduct                2 USB-Serial Controller
>   iSerial                 0
>   bNumConfigurations      1

Johan