HAL_Linux: implement uart timestamp API

libraries/AP_HAL_Linux/UARTDriver.cpp

@@ -87,6 +87,8 @@ void UARTDriver::begin(uint32_t b, uint16_t rxS, uint16_t txS)
 
     _device->set_speed(b);
 
+    _baudrate = b;
+
     _allocate_buffers(rxS, txS);
 }
 
@@ -471,6 +473,10 @@ void UARTDriver::_timer_tick(void)
         }
         _readbuf.commit((unsigned)ret);
 
+        // update receive timestamp
+        _receive_timestamp[_receive_timestamp_idx^1] = AP_HAL::micros64();
+        _receive_timestamp_idx ^= 1;
+        
         /* stop reading as we read less than we asked for */
         if ((unsigned)ret < vec[i].len) {
             break;
@@ -479,3 +485,27 @@ void UARTDriver::_timer_tick(void)
 
     _in_timer = false;
 }
+
+/*
+  return timestamp estimate in microseconds for when the start of
+  a nbytes packet arrived on the uart. This should be treated as a
+  time constraint, not an exact time. It is guaranteed that the
+  packet did not start being received after this time, but it
+  could have been in a system buffer before the returned time.
+  
+  This takes account of the baudrate of the link. For transports
+  that have no baudrate (such as USB) the time estimate may be
+  less accurate.
+  
+  A return value of zero means the HAL does not support this API
+*/
+uint64_t UARTDriver::receive_time_constraint_us(uint16_t nbytes) const
+{
+    uint64_t last_receive_us = _receive_timestamp[_receive_timestamp_idx];
+    if (_baudrate > 0) {
+        // assume 10 bits per byte.
+        uint32_t transport_time_us = (1000000UL * 10UL / _baudrate) * nbytes;
+        last_receive_us -= transport_time_us;
+    }
+    return last_receive_us;
+}

libraries/AP_HAL_Linux/UARTDriver.h

@@ -49,12 +49,28 @@ public:
        _device->set_flow_control(flow_control_setting);
    }
 
+    /*
+      return timestamp estimate in microseconds for when the start of
+      a nbytes packet arrived on the uart. This should be treated as a
+      time constraint, not an exact time. It is guaranteed that the
+      packet did not start being received after this time, but it
+      could have been in a system buffer before the returned time.
+
+      This takes account of the baudrate of the link. For transports
+      that have no baudrate (such as USB) the time estimate may be
+      less accurate.
+
+      A return value of zero means the HAL does not support this API
+     */
+    uint64_t receive_time_constraint_us(uint16_t nbytes) const override;
+    
 private:
     AP_HAL::OwnPtr<SerialDevice> _device;
     bool _nonblocking_writes;
     bool _console;
     volatile bool _in_timer;
     uint16_t _base_port;
+    uint32_t _baudrate;
     char *_ip;
     char *_flag;
     bool _connected; // true if a client has connected
@@ -66,6 +82,10 @@ private:
     AP_HAL::OwnPtr<SerialDevice> _parseDevicePath(const char *arg);
     uint64_t _last_write_time;
 
+    // timestamp for receiving data on the UART, avoiding a lock
+    uint64_t _receive_timestamp[2];
+    uint8_t _receive_timestamp_idx;
+    
 protected:
     const char *device_path;
     volatile bool _initialised;