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Fix the unsigned underflow in calcMaxPacketMillis()'s airtime fallback #12

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Fix the unsigned underflow in calcMaxPacketMillis()'s airtime fallback #12

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25 changes: 24 additions & 1 deletion src/helpers/radiolib/RadioLibWrappers.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -11,6 +11,14 @@
#define NUM_NOISE_FLOOR_SAMPLES 64
#define SAMPLING_THRESHOLD 14

// Payload budget calcMaxPacketMillis() assumes when the modem cannot tell it
// how long a packet takes. Long on purpose: this deadline exists to break a
// stuck header IRQ, and one that expires early would clear the flags of a
// packet still arriving.
#ifndef MAX_PACKET_FALLBACK_PAYLOAD_US
#define MAX_PACKET_FALLBACK_PAYLOAD_US 4000000UL
#endif

static volatile uint8_t state = STATE_IDLE;

// this function is called when a complete packet
Expand Down Expand Up @@ -252,7 +260,22 @@ PacketMillis RadioLibWrapper::calcMaxPacketMillis(uint8_t sf, float bw, uint8_t
// airtime for max packet at current radio settings
uint32_t total_us = _radio->getTimeOnAir(MAX_TRANS_UNIT);
// airtime for payload only (no preamble, header or SOF)
uint32_t payload_us = total_us > preamble_us ? total_us - preamble_us : 4000 - preamble_us; // fallback to 4 secs at worst case
uint32_t payload_us;
if (total_us > preamble_us) {
payload_us = total_us - preamble_us;
} else {
// getTimeOnAir() gave nothing usable (it returns 0 on an unconfigured
// modem). Fall back to the 4 s this has always claimed -- as 4 s of
// *payload*, not as 4 s of total airtime minus the preamble.
//
// The value used to be 4000, i.e. 4 ms, and the subtraction underflowed for
// any setting whose preamble exceeds that: every one of them. An underflow
// here is not a mis-sized deadline but the absence of one, because the
// result becomes a ~49-day payload watchdog, so CustomSX1262::isReceiving()
// would hold a latched HEADER_VALID true forever and isReceiving() would
// never again report the channel idle.
payload_us = MAX_PACKET_FALLBACK_PAYLOAD_US;
}
// rescale payload_us for max possible CR
if (cr >= 5 && cr < 8) { payload_us = (payload_us * 8) / cr; }

Expand Down
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