SX126x: keep a staged-over frame's RX_DONE across the resume

A CAD skipped because a frame was in flight set keepRxIrqsAtResume only
when RX_DONE or CRC_ERR was already up. With a header but no RX_DONE yet,
the resume cleared RX_DONE, CRC_ERR, HEADER_ERR and TIMEOUT - and on a
CH341 host the flag read in stageTxInRx() and that clear are a USB round
trip apart, so a frame completing in between lost its RX_DONE. The poll
then had no edge to deliver and the frame waited for checkStaleRxFlags()
to retire the latched HEADER_VALID, one max packet later.

Keep RX_DONE and CRC_ERR for every busy verdict, so the resume's
checkRxDoneIrqFlag() drives the readout. HEADER_ERR and TIMEOUT are still
cleared unconditionally: no readout waits on them, and a stale one costs
checkStaleRxFlags() a re-arm. Only RX_DONE is mapped to DIO1, so leaving
the other terminal flag latched holds no interrupt line.

Found by CodeRabbit on #12016; the blanket keep it proposed would have left
a stale HEADER_ERR or TIMEOUT latched instead.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0112S3QXU8WUGSCcMQHnwRKZ

src/mesh/SX126xInterface.cpp

@@ -543,12 +543,15 @@ template <typename T> bool SX126xInterface<T>::resumeRunningReceive()
 #endif
     if (!rxArmedContinuous)
         return false;
-    // readData() clears these, but handleReceiveInterrupt()'s early outs do not, and a latched one would hold DIO1
-    // high past the re-arm. PREAMBLE/HEADER_VALID stay: they may belong to the next frame, already arriving. A scan
-    // skipped for an unread frame keeps them all, for its readout.
+    // readData() clears these, but handleReceiveInterrupt()'s early outs do not. PREAMBLE/HEADER_VALID stay: they
+    // may belong to the next frame, already arriving. A scan skipped for a frame in flight or unread keeps that
+    // frame's RX_DONE (the only flag on DIO1) and CRC_ERR for its readout, which checkRxDoneIrqFlag() below drives -
+    // including an RX_DONE that landed in the USB round trip since the scan read the flags. A stale HEADER_ERR or
+    // TIMEOUT has no readout waiting and would cost checkStaleRxFlags() a re-arm, so it goes either way.
+    uint32_t clearMask = RADIOLIB_SX126X_IRQ_HEADER_ERR | RADIOLIB_SX126X_IRQ_TIMEOUT;
     if (!keepRxIrqs)
-        lora.clearIrqFlags(RADIOLIB_SX126X_IRQ_RX_DONE | RADIOLIB_SX126X_IRQ_CRC_ERR | RADIOLIB_SX126X_IRQ_HEADER_ERR |
-                           RADIOLIB_SX126X_IRQ_TIMEOUT);
+        clearMask |= RADIOLIB_SX126X_IRQ_RX_DONE | RADIOLIB_SX126X_IRQ_CRC_ERR;
+    lora.clearIrqFlags(clearMask);
     activeReceiveStart = 0; // as the standby this replaces would
     RadioLibInterface::startReceive();
     enableInterrupt(isrRxLevel0);
@@ -779,8 +782,9 @@ template <typename T> bool SX126xInterface<T>::stageTxInRx()
         // Its readout checks whether our bytes can lie across it
         if (irq & (RADIOLIB_SX126X_IRQ_HEADER_VALID | doneIrqs))
             noteStagedOverFrame(base, numbytes);
-        // The busy verdict's rearmReceive() resumes the RX: it must leave a finished frame's flags for its readout
-        keepRxIrqsAtResume = (irq & doneIrqs) != 0;
+        // The busy verdict's rearmReceive() resumes the RX: it must leave the frame's terminal flags for its
+        // readout, whether it had already finished or finishes while we get there
+        keepRxIrqsAtResume = true;
         LOG_DEBUG("TX stage in RX: frame arriving or unread (irq 0x%04x), scan skipped", (unsigned)irq);
         return true;
     }

src/mesh/SX126xInterface.h

@@ -120,7 +120,7 @@ template <class T> class SX126xInterface : public RadioLibInterface
     uint8_t rxClobberBase = 0;
     size_t rxClobberLen = 0;
     uint8_t rxClobberBytes[256];
-    /** That frame had finished: the next resumeRunningReceive() must not clear its IRQ flags before it is read */
+    /** A frame was in flight or unread: the next resumeRunningReceive() keeps RX_DONE/CRC_ERR for its readout */
     bool keepRxIrqsAtResume = false;
     /** A payload written during its backoff: its length (0 if none), packet id, offset and bytes */
     size_t earlyStagedLen = 0;