drivers/sx127x: reduce use of floats

[maribu]

Contribution description

Convert the floating point arithmetic to integer arithmetic.

Beware: I neither tested this nor did I try to understand the calculations.

Testing procedure

• air time computation should yield roughly the same result
• frequency / channel computation should yield roughly the same result

Issues/PRs references

#19614

[jia200x]

The original Semtech pkg hardcoded this information in the driver. In practice it doesn't make sense, as it is specific to the MAC config and has nothing to do with the transceiver.
GNRC LoRaWAN uses a lookup table based implementation to calculate that. Shouldn't we simply deprecate getting time on air from the driver and just call that function directly from the pkg contrib?

[maribu]

Agreed. But I don't think we need to deprecate this before removing as I see this as an internal API.

Would you mind to PR this? I can rebase the channel computation on top of that.

[maribu]

I updated this now to only do the straight forward conversions in the hope that the airtime computation is about to get dropped anyway.

[jia200x]

I updated this now to only do the straight forward conversions in the hope that the airtime computation is about to get dropped anyway.

Let's just merge this one then. I realized the lora_time_on_air function does not consider using other BW values. This may be a problem for certain LoRaWAN regions. Although it is easy to fix, it requires to extend the function.

[maribu]

It still needs ACK, though 😉

[jia200x]

sure, I would just like to test it before

[jia200x]

ACK. The new function yields the same values as the float version.

I noticed that this function seems to overestimates the ToA for low pkt_len values. But this was already the case with the float version and it shouldn't be a problem for LoRaWAN

[kfessel]

you may want to add to comment this is 32MHz/2^19

[maribu]

I added the comment @kfessel suggested and the white space changes uncrustify preferred. (So only white space changes from a C compiler's point of view since the last testing.)

Should be good to go now.

[maribu]

bors merge

[riot-ci]

Murdock results

✔️ PASSED

826095d drivers/sx127x: reduce use of floats

Success	Failures	Total	Runtime
6934	0	6934	11m:42s

Artifacts

• Documentation preview

[benpicco]

bors merge

[bors]

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• Murdock
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