[Subsurface-divelog/libdc] Sync with Jef's upstream (#6)

[Linus Torvalds]

On Thu, May 25, 2017 at 11:35 AM, Jef Driesen <jef at libdivecomputer.org> wrote:
>
>
> The explanation is in the commit message:
>
>     Note that the interpretation of the PPO2 status bit appears to be
>     reversed (0=external and 1=internal).

You mis-understand. It's not the particular status bit polarity I
worry about. It's simply that whole "average PPO2 data" piece.

IOW, this code:

  #ifdef SENSOR_AVERAGE
        sample.ppo2 = data[offset + 6] / 100.0;
        if (callback) callback (DC_SAMPLE_PPO2, sample, userdata);
  #else

fundamentally makes no sense as it is now.

There are two possibilities I see for that sample.ppo2 value:

 (a) it too only exists for the internal sensor case

 (b) it exists even when we have an external sensor, and then contains
the single PPO2 value that the external sensor gives us.

but the code right now is wrong in *either* case.

So if it's case (a), then that whole #ifdef SENSOR_AVERAGE logic
should be _inside_ the PPO2_EXTERNAL check.

And if it is case (b), then we should have an "else" case for that
PPO2_EXTERNAL case, and give the user that single PPO2 value.

See what I'm saying? As it is, the code makes no sense and absolutely
cannot be right.

                 Linus

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