drivers/at86rf2xx: support frame retry reporting on AT86RFR2

[benpicco]

Contribution description

AT86RFR2 doesn't report retries directly, but it can generate a TX start interrupt for each (re)try, so we can count manually.

Testing procedure

Currently retry count is not used anywhere in RIOT.
So you will either need #14448 or you manually add

--- a/drivers/at86rf2xx/at86rf2xx_netdev.c
+++ b/drivers/at86rf2xx/at86rf2xx_netdev.c
@@ -696,6 +696,8 @@ static void _isr_send_complete(at86rf2xx_t *dev, uint8_t trac_status)
                       AT86RF2XX_XAH_CTRL_2__ARET_FRAME_RETRIES_OFFSET;
 #endif
 
+    printf("retries: %d\n", dev->tx_retries);
+
     DEBUG("[at86rf2xx] EVT - TX_END\n");
 
     if (netdev->event_callback && (dev->flags & AT86RF2XX_OPT_TELL_TX_END)) {

to check that retries are reported correctly.

Issues/PRs references

Missing feature from #9172

[fjmolinas]

@aabadie has the hardware and could test!

[aabadie]

Testing this PR with examples/gnrc_networking on atmega256rfr2-xpro (and after applying the diff above) and got this output when pinging an unreachable address:

> ping fe80::dd22:84f6:2f1f:2d58
2020-09-08 14:13:29,791 #  ping fe80::dd22:84f6:2f1f:2d58
2020-09-08 14:13:29,808 # retries: 3
2020-09-08 14:13:30,807 # retries: 3
2020-09-08 14:13:31,806 # retries: 3
2020-09-08 14:13:32,792 # 
2020-09-08 14:13:32,796 # --- fe80::dd22:84f6:2f1f:2d58 PING statistics ---

No idea if this is expected or not.

[benpicco]

If it gets no ACK is will re-try as often as you have configured it to do.

[aabadie]

If it gets no ACK is will re-try as often as you have configured it to do.

Thanks but sorry, I don't know about it that much. Do you have something more precise ?

[jia200x]

I don't have the hardware with me, but conceptually makes sense to me

[benpicco]

Thanks but sorry, I don't know about it that much. Do you have something more precise ?

The radio will send the the frame a number of times until it receives an ACK from the destination if the ACK requested flag is set.
The max. Retrans. in ifconfig will tell you how often it will attempt to re-transmit a packet before it gives up.

If you send a packet to a node that doesn't exist, the radio will try that many times to deliver the packet.

[aabadie]

So I guess the output was correct.

[aabadie]

ACK