AP_Logger, add dBm unit to log structure

[olliw42]

Summary

I kindly ask to add a unit specifier for dBm to the log structure. It may not be used inside ArduPilot code, but Lua scripts are allowed to define their own datalog messages, where it can be highly useful. The specific application is mLRS, which provides an ArduPilot Lua script which allows us to log substantial link performance and link information metrics, among which are lots of "dBm types", such as rssi, snr and power.

Classification & Testing (check all that apply and add your own)

• [ x] Checked by a human programmer
• Non-functional change
• No-binary change
• Infrastructure change (e.g. unit tests, helper scripts)
• Automated test(s) verify changes (e.g. unit test, autotest)
• Tested manually, description below (e.g. SITL)
• Tested on hardware
• Logs attached
• Logs available on request

Description

[olliw42]

I am sorry for the force pushes, I tried to change the commit message to start with AP_Logger: xxxx, but my efforst failed. Don't knwo what to do else.

[rmackay9]

While we don't normally have code in AP that isn't used, my guess is we can make an exception because of the lua script mentioned. I think the next question is just about whether "R" is the right letter to use. It's a bit arbitrary but we normally pick a letter that's related somehow.

[rmackay9]

After a quick chat with @timtuxworth "R" for "rssi" seems OK to me. Other letters like "d", "D", "b", "B" and "m" are already taken. "M" is free but that doesn't seem any better.

[timtuxworth]

Seems like a good idea to me - I see "R" for RSSI makes sense.

[olliw42]

my line of thoughts was, 'd', 'D' (for dBm), 'p', 'P' (for power) are taken, so maybe 'R' alluding to RF or RADIO or ... well, rssi

we don't normally have code in AP that isn't used

I know that well and perfectly understand that.
But Lua scripts exist (and the mentioned one is not the first and only using logger). An alternative could be to provide a lua means and a dataflash log means (new FMT ??) to record user supplied units. Would be more generic, also would help viewers, and avoid that, but is way more complicated.

[rmackay9]

Hi @olliw42,

Could you rebase on master? We don't accept merge commits into AP because we find it makes backporting more difficult - we prefer a linear history.

[peterbarker]

When choosing letters, try to stay away from SI unit shortenings.

R looks pretty safe.

[olliw42]

ah ... it offered me a button to update to main ... sorry for clicking. Corrected.

[olliw42]

MANY THX!
this was easy and smooth

will this appear in AP 4.8, or may it be backported, e.g to 4.7.2?