Cochran Changes

[John Van Ostrand]

On Wed, Jan 27, 2016 at 10:21 AM, Jef Driesen <jef at libdivecomputer.org>
wrote:

> On 2016-01-20 18:41, John Van Ostrand wrote:
>
>> I think this is a summary of where we left of:
>>
>> 1. dc_context_t pointer doesn't need to be passed to cochran serial open a
>> setup functions. (fixed with patch pending.)
>>
>
> Patch looks fine to me.
>
> 2. Cochran can store more log entries than profiles. So when the profile
>> ring buffer wraps earlier logs will point to profile data from other
>> dives.
>> Jef suggests processing dives in reverse chronological order, adding up
>> profile data used and not processing profiles after it's processed a full
>> ring-buffer of data. I have yet to work on this or consider alternatives.
>>
>
> The dc_device_foreach() function must return the dives in reverse
> chronological order. This is a requirement for the download only new dives
> feature. When dives are returned in reverse chronological order, an
> application (or libdivecomputer itself) can simply stop downloading dives
> as soon as a previously downloaded dive is recognized. Very simple and
> efficient.
>

This is my loop in dc_device_foreach.

    int i;
    for (i = data->dive_count - 1; i > data->fp_dive_num; i--) {



> Thus if we need to return the dives in reverse chronological order, it
> makes sense to process (and also download) the data in this order.
> Otherwise you'll end up with a rather inefficient implementation.
>
> I have to read through your documentation again, but processing the dives
> in reverse order might also make the recovery of corrupt dives easier. If
> the tail of a dive is missing, then it can run at most until the start of
> the next dive. And due the reverse order we already have that one.
>
> 3. Corrupt dive handling. In some cases (like a low battery especially in
>> cold water) the computer resets during a dive. This results in a
>> "start-dive" block written but no valid "end-dive" block written. We know
>> information from the start of a dive (like date/time, gasses, profile
>> start
>> pointer, etc.) but we don't know information accumulated during or at the
>> end of a dive (like end-profile pointer, max depth, min temp, etc.) I've
>> taken to guessing the end of a dive by starting with the next dive's
>> pre-dive-profile-pointer and backing up until we think we have the
>> previous
>> dive's end. We haven't resolved our differences on this. It seems to down
>> to the question: Do we present a partial or broken profile in the interest
>> of giving the diver something or do we give nothing in the interest of
>> being accurate?
>>
>
> That's a difficult question. In general, I prefer to be very strict and
> simply fail on unexpected data. Usually this is the correct thing to do,
> because such unexpected data often turns out to be an wrong assumption in
> the code. So being strict helps finding bugs. But sometimes the data is
> really wrong (due to a firmware bug, running out of battery during the
> dive, etc), and if it happens frequently, then a workaround might indeed be
> necessary.
>
> It also depends on where the data "corruption" is located. If the
> information needed to move from one dive to another is good, but we are
> unsure about the contents of the dive, then we can return the bogus dive to
> the application and let the parser deal with it. You might get incorrect
> data for that particular dive, or even a failure to parse the dive. This
> would ensure that we can still download the other dives. But if the primary
> structure is damaged and we can no longer safely move to the next dive,
> then I think we should fail.
>
> Jef
>

I've been working on code to retrieve corrupt dives and it's becoming
unwieldy. In working backwards from a future dive's profile I have to
remove inter-dive events which vary in length. it's possible that two
events might match the data and the code would have to decide which event
to use. The code also needs to do range checking, like ensuring it doesn't
exceed the malloc'ed memory and things like ring-buffer wrapping. I should
also remove any surface time (the time the DC still stores samples in case
you re-descend.)

There are ways to make this better but I think I'm going to have to think
about it for a bit. I'm considering pulling that code. Any ideas on what I
should do with the code so it isn't lost. An #ifdef maybe?
-- 
John Van Ostrand
At large on sabbatical
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