[ncp] fix logic surrounding the call to otPlatWakeHost (#2208)

Due to the nature of how frames can be processed in ncp_uart.cpp, it is possible for a call to otPlatWakeHost to execute before the SPINEL_PROP_HOST_POWER_STATE response message has been sent to the host. This order is unexpected and not sufficient for proper operation. Instead the call to otPlatWakeHost should only occur after the SPINEL_PROP_HOST_POWER_STATE response message has been sent and only when a new subsequent message has been made ready for delivery over the UART.

To solve this, the change below detects when the SPINEL_PROP_HOST_POWER_STATE response message has been completely written to mUartBuffer and immediately calls otPlatUartSend() when this condition is met. With this change the expected order of operations can be maintained.

src/ncp/ncp_base.cpp

@@ -534,6 +534,7 @@ NcpBase *NcpBase::sNcpInstance = NULL;
 NcpBase::NcpBase(otInstance *aInstance):
     mInstance(aInstance),
     mTxFrameBuffer(mTxBuffer, sizeof(mTxBuffer)),
+    mHostPowerStateInProgress(false),
     mLastStatus(SPINEL_STATUS_OK),
     mSupportedChannelMask(OT_RADIO_SUPPORTED_CHANNELS),
     mChannelMask(OT_RADIO_SUPPORTED_CHANNELS),
@@ -545,7 +546,6 @@ NcpBase::NcpBase(otInstance *aInstance):
     mThreadChangedFlags(0),
     mChangedPropsSet(),
     mHostPowerState(SPINEL_HOST_POWER_STATE_ONLINE),
-    mHostPowerStateInProgress(false),
     mHostPowerReplyFrameTag(NcpFrameBuffer::kInvalidTag),
     mHostPowerStateHeader(0),
 #if OPENTHREAD_CONFIG_NCP_ENABLE_PEEK_POKE

src/ncp/ncp_base.hpp

@@ -639,6 +639,7 @@ protected:
     static uint8_t LinkFlagsToFlagByte(bool aRxOnWhenIdle, bool aSecureDataRequests, bool aDeviceType, bool aNetworkData);
     otInstance *mInstance;
     NcpFrameBuffer  mTxFrameBuffer;
+    bool mHostPowerStateInProgress;
 
 private:
     enum
@@ -669,7 +670,6 @@ private:
     uint32_t mChangedFlags;
     bool mShouldSignalEndOfScan;
     spinel_host_power_state_t mHostPowerState;
-    bool mHostPowerStateInProgress;
     NcpFrameBuffer::FrameTag mHostPowerReplyFrameTag;
     uint8_t mHostPowerStateHeader;
 

src/ncp/ncp_uart.cpp

@@ -99,6 +99,7 @@ NcpUart::NcpUart(otInstance *aInstance):
     mUartBuffer(),
     mState(kStartingFrame),
     mByte(0),
+    mUartSendImmediate(false),
     mUartSendTask(*aInstance, EncodeAndSendToUart, this)
 {
     mTxFrameBuffer.SetFrameAddedCallback(HandleFrameAddedToNcpBuffer, this);
@@ -136,6 +137,7 @@ void NcpUart::EncodeAndSendToUart(Tasklet &aTasklet)
 void NcpUart::EncodeAndSendToUart(void)
 {
     uint16_t len;
+    bool prevHostPowerState;
 
     while (!mTxFrameBuffer.IsEmpty() || (mState == kFinalizingFrame))
     {
@@ -164,8 +166,22 @@ void NcpUart::EncodeAndSendToUart(void)
                 SuccessOrExit(mFrameEncoder.Encode(mByte, mUartBuffer));
             }
 
+            // track the change of mHostPowerStateInProgress by the 
+            // call to OutFrameRemove.
+            prevHostPowerState = mHostPowerStateInProgress;
+
             mTxFrameBuffer.OutFrameRemove();
 
+            if (prevHostPowerState && !mHostPowerStateInProgress)
+            {
+                // If mHostPowerStateInProgress transitioned from true -> false
+                // in the call to OutFrameRemove, then the frame should be sent
+                // out the UART without attempting to push any new frames into
+                // the mUartBuffer. This is necessary to avoid prematurely calling
+                // otPlatWakeHost.
+                mUartSendImmediate = true;
+            }
+
             mState = kFinalizingFrame;
 
             // fall through
@@ -175,6 +191,13 @@ void NcpUart::EncodeAndSendToUart(void)
             SuccessOrExit(mFrameEncoder.Finalize(mUartBuffer));
 
             mState = kStartingFrame;
+
+            if (mUartSendImmediate)
+            {
+                // clear state and break;
+                mUartSendImmediate = false;
+                break;
+            }
         }
     }
 

src/ncp/ncp_uart.hpp

@@ -114,6 +114,7 @@ private:
     UartTxState     mState;
     uint8_t         mByte;
     uint8_t         mRxBuffer[kRxBufferSize];
+    bool            mUartSendImmediate;
     Tasklet         mUartSendTask;
 };