uhd: enable priority scheduling on receive loop

Though the receive loop ultimately drives the GSM clock,
it does not have any priority because it runs as a
separate thread from the trasmit loop. The transmit
has priority because it starts the UHD device, where
priority scheduling is enabled. The result is frequent
underruns, which occur regardless of buffer size tuning.

To address this, break out and expose the priority
setting so that it can be called from the radioInterface
at the start of a new thread.

Tested on a modest Intel Core 2 Duo tablet running
Linux 2.6.33.7.2-rt30, this reduced underruns down to
near zero.

Signed-off-by: Thomas Tsou <ttsou@vt.edu>

public-trunk/Transceiver/Device.h

@@ -39,6 +39,7 @@ public:
 	virtual bool open() = 0;
 	virtual bool start() = 0;
 	virtual bool stop() = 0;
+	virtual void setPriority() = 0;
 
 	/** Read samples from the USRP.
 	    @param buf preallocated buf to contain read result

public-trunk/Transceiver/UHDDevice.cpp

@@ -135,6 +135,7 @@ public:
 	bool open();
 	bool start();
 	bool stop();
+	void setPriority();
 
 	int readSamples(short *buf, int len, bool *overrun, 
 			TIMESTAMP timestamp, bool *underrun, unsigned *RSSI);
@@ -286,8 +287,7 @@ bool UHDDevice::start()
 		return false;
 	}
 
-	// Enable priority scheduling
-	uhd::set_thread_priority_safe();
+	setPriority();
 
 	// Start asynchronous event (underrun check) loop
 	asyncEventServiceLoopThread.start(
@@ -322,6 +322,12 @@ bool UHDDevice::stop()
 }
 
 
+void UHDDevice::setPriority()
+{
+	uhd::set_thread_priority_safe();
+	return;
+}
+
 int UHDDevice::readSamples(short *buf, int len, bool *overrun,
 			TIMESTAMP timestamp, bool *underrun, unsigned *RSSI)
 {

public-trunk/Transceiver/USRPDevice.h

@@ -185,6 +185,9 @@ private:
   /** Stop the USRP */
   bool stop();
 
+  /** Set priority not supported */
+  void setPriority() { return; }
+
   /**
 	Read samples from the USRP.
 	@param buf preallocated buf to contain read result

public-trunk/Transceiver/radioInterface.cpp

@@ -276,6 +276,11 @@ bool RadioInterface::tuneRx(double freq)
   return usrp->setRxFreq(freq);
 }
 
+void RadioInterface::setPriority()
+{
+  usrp->setPriority();
+  return;
+}
 
 void RadioInterface::start()
 {
@@ -305,6 +310,8 @@ void *TransmitRadioServiceLoopAdapter(RadioInterface *radioInterface)
 
 void *ReceiveRadioServiceLoopAdapter(RadioInterface *radioInterface)
 {
+  radioInterface->setPriority();
+
   while (1) {
     radioInterface->driveReceiveRadio();
     pthread_testcancel();

public-trunk/Transceiver/radioInterface.h

@@ -189,6 +189,9 @@ public:
   /** set receive frequency */
   bool tuneRx(double freq);
 
+  /** set thread priority */
+  void setPriority();
+
 protected:
 
   /** drive transmission of GSM bursts */