Subsurface Mobile: Adding mares USB (cp210x) support; how to proceed

[Dirk Hohndel]

> On Mar 1, 2020, at 10:32 AM, Christof Arnosti <charno at charno.ch> wrote:
> 
> Soo... I own a Mares Puck Pro, and I want to use Subsurface Mobile to
> transfer dives from the computer. 
> 
> I found that currently for android a libftdi / libusb-based driver
> (serial_ftdi.{c,h}) is available for FTDI based serial converters.

And by 'available' we mean:

IT DOESN'T WORK ON BASICALLY ANY ANDROID DEVICE

This works on some old Android devices. And it can be hacked to work
on rooted Android devices. But for 90+% of our users this doesn't work
at all. As you point out below, we have made attempts to fix this and 
admittedly I have spent way too much time on that already... there's even
a PR on GitHub where I try to make this work but I got stuck...

> The Mares USB cable uses a cp210x based converter.
> 
> In the FAQ at subsurface-divelog.org I found the following paragraph:
> 
> "Subsurface-mobile on Android and dive computers with FTDI download cables
> 
> Subsurface-mobile on Android (but not on iOS beacuse of hardware
> limitations) is designed to be able to download from some dive computers
> using FTDI based download cables and a USB OTG adapter. Unfortunately,
> there is an issue with the way in which Subsurface-mobile opens USB
> devices on Android. We do this from “native code”, not from a Java
> application. And while many Android phones allow this method of
> accessing OTG devices, an (apparently increasing) number of devices
> don’t. On these Android devices the attempt to download from
> divecomputers via the USB OTG adapter always fails. We understand in
> theory how to work around this problem, but haven’t found the right
> developer who could volunteer their time to help us fix the issue."

I need to update the FAQ. Initially it was indeed "some" where it failed.
Now it is basically "all".

> Well... Maybe I want to be this person, and for sure I want to be the
> person to add cp210x support. I would be glad if somebody could explain
> what the proposed workaround mentioned in the paragraph would be?

Paging Anton - but I know that he has explained it on this mailing list a
few times in the past, so googling for posts from him with Android in the
title might get you to the right one.

> While recherching I found the git repo
> https://github.com/mik3y/usb-serial-for-android containing a collection
> of USB serial converter drivers for android. The drivers are licensed as
> LGPL2.1 (GPL compatible) and are written in Java using the Android USB
> Host API.
> 
> From the code that I have read I have two possible ways to go forward:
> 
> First (my preferred way):
> Directly use the java code from usb-serial-for-android, write a small
> abstraction class in java to provide a convenient interface, and a small
> class in c to call the java abstraction class (similar to what
> serial_ftdi does with libftdi).
> 
> Pros:
> - This could act as a generic way to access USB serial-based
> divecomputers, including FTDI ones.
> - Using the official Android USB Host API means that the chance for
> support on different phones is quite high.
> 
> Cons:
> - More Platform-Specific, non-C code.
> 
> Second:
> Translate the CP210x driver from usb-serial-for-android to C code using
> libusb. 
> 
> Pros:
> - C-Code
> 
> Cons:
> - More work
> - Chipset-Specific
> 
> What do you think about this two possibilities? I would prefer to
> directly use usb-serial-for-android, since for the desktop application
> there is already existing support for the different USB-Serial chipsets,
> so portability of the code for non-android builds is not that important
> in my eyes. With this subsurface would also only need one
> android-specific driver implementation for different chipsets.
> 
> I would love to hear your toughts and maybe get some additional pointers
> I could use while developing. I would mainly work on the task in the
> next week, since I have holidays and the event I wanted to go to was
> canceled due to the corona virus.

Since Anton knows more about this than anyone else on this list (or at
least I think so), I really hope that tagging him directly on the To: line will
help to catch his attention. It seems much more useful to have him respond...

Thanks for looking into this. It would be amazing to see this fixed. I had
basically given up hope...

/D