[riot-notifications] [RIOT-OS/RIOT] kw2xrf: CSMA support missing in driver (#10364)

Thomas Stilwell notifications at github.com
Thu Sep 12 06:15:08 CEST 2019

I was puzzled at first by the timer usage too. These radios have a tighter integration with software than others I've used. The use of the radio timer isn't just to fire callbacks in software. It's actually used to trigger events in the radio hardware like timing out an ACK wait for example.

In other words:
> Why isn't the timer just set to `KW41ZRF_ACK_WAIT_TIME` AFTER the hardware has signaled us, that the TX operation is complete?

Because the hardware waits to signal us the outcome of the TX operation until it has received an ACK or not, and the timer is what causes it to stop waiting for a missed ACK and signal us. At least that's the case with kw41zrf. It looks like kw2xrf does as you say where the hardware signals TX complete before waiting for ACK and only then the driver starts an ACK timer. It's possible that both radios could support either paradigm and the difference in the two drivers is just developers' preference.

My interpretation of NETOPT_AUTOCCA is that the clear channel assessment should be performed but NOT the retries. Meaning if CCA succeeds then the packet is transmitted but if it fails then the radio driver returns a medium-busy failure rather than retrying.

I interpret NETOPT_CSMA to mean NETOPT_AUTOCCA plus retries. And you're right the documentation specifically says that it must be hardware CSMA support which is interesting. Is that because we have (or intend to have (or used to indend to have)) software CSMA retries implemented in an upper layer? And maybe it's getting implemented in netdev drivers because the upper layer implementation is missing so far? I haven't looked in to that.

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