Can't Import dives from Mares Quad Air

[Linus Torvalds]

On Mon, Jan 11, 2021 at 11:37 AM Markus Hinkelmann via subsurface
<subsurface at subsurface-divelog.org> wrote:
>
> I tried to import my dives from my Mares Quad Air using the Interface „Mares BLUELINK Pro“ to subsurface.

I'm afraid that the Mares BlueLink Pro has been the most problematic
piece of BLE hardware we have ever seen.

It has been known to work - often very very slowly - but it has always
been extremely flaky. I've never figured out why.

> The computer was able to connect to the interface using the mode „BT-classic“ but the import fails.

I didn't even know that it supported BT-classic at all. I've only ever
seen it do BLE.

I was going to say that BT-classic cannot possibly work, but:

> A dump file was not created, but I was able to create a libdivecomputer log file (see attachment).

Strangely, it actually partially does seem to have worked. There are a
few valid reply packets with the valid '0xAA' packet reply byte, and
the communication actually starts. Color me I'm surprised.

So that log shows us sending several commands to it, and starting to
do a data transfer.  Yeah, we get a few NAK packets (0xEA) back
instead of 0xAA, and a couple of "no reply at all" cases, but even
then the re-try ends up working a few times.

But then at some point, even when retrying, it just returns an
all-ones (0xff) answer, and after five failres for that command we
just give up.

Honestly, this _might_ be one of those cases where "if you try again,
eventually it might work".

But equally honestly, I've pretty much given up on the Mares BlueLink
adapter. I don't know why it's so flaky. The Mares mobile app seems to
be able to talk to it, but when I did a packet trace, I didn't see it
do anything special, so I don't know what is going on. It might be
some very subtle timing issue that the Mares app is aware of.

            Linus