Rover: fix WHEEL_DISTANCE timestamp overflow

[peterbarker]

Summary

Fixes an integer overflow when sending the WHEEL_DISTANCE telemetry message

Testing (more checks increases chance of being merged)

• Checked by a human programmer
• Tested in SITL
• Tested on hardware
• Logs attached
• Logs available on request
• Autotest included

Description

The time_usec field (uint64_t, units=us) was computed as 1000UL * AP_HAL::millis(). On ARM, UL is 32-bit, so the multiplication overflows after ~71 minutes and the 32-bit result is then zero-extended to 64 bits, producing a garbage timestamp.

Use AP_HAL::micros64() which returns a proper 64-bit microsecond value.

Tested with the following patch:

--- a/ArduPlane/Plane.cpp
+++ b/ArduPlane/Plane.cpp
@@ -392,6 +392,23 @@ void Plane::one_second_loop()
     rollController.set_notch_sample_rate(loop_rate);
     pitchController.set_notch_sample_rate(loop_rate);
     yawController.set_notch_sample_rate(loop_rate);
+
+    // Demonstrate WHEEL_DISTANCE timestamp overflow (fixed in Rover).
+    // On 32-bit ARM sizeof(unsigned long)==4, so 1000UL * millis() is a
+    // 32-bit multiplication that overflows after ~71 minutes.  Use a fixed
+    // sample value to make the result predictable:
+    //   sample_ms = 5000000  (~83 minutes of uptime)
+    //   correct usec         = 5000000000  (high-32 = 1)
+    //   1000UL * sample_ms   = 705032704   (high-32 = 0, data lost)
+    {
+        const uint32_t sample_ms = 5000000;              // ~83 minutes
+        const uint64_t truncated = 1000UL  * sample_ms;  // 32-bit mul: 705032704
+        const uint64_t correct   = 1000ULL * sample_ms;  // 64-bit mul: 5000000000
+        gcs().send_text(MAV_SEVERITY_INFO,
+                        "WD ts overflow: truncated=%llu correct=%llu",
+                        (unsigned long long)truncated,
+                        (unsigned long long)correct);
+    }
 }

That patch on a ZeroOneX6:

AP: WD ts overflow: truncated=705032704 correct=5000000000

Reformatted:

truncated   705032704
correct    5000000000

[rmackay9]

Great, txs for the fix.

Do we want to get Claude Sonnet co-author credit? :-)

[peterbarker]

Do we want to get Claude Sonnet co-author credit? :-)

It's co-authored on the patch already :-)

[rmackay9]

Hi @peterbarker, indeed, that's why I brought it up.. maybe I should have added a "really" into the sentence..