Modified CAN driver API: Passing the next pending TX frames into the select() call to facilitate proper prioritization in the driver

libuavcan/include/uavcan/driver/can.hpp

@@ -13,6 +13,11 @@
 
 namespace uavcan
 {
+/**
+ * This limit is defined by the specification.
+ */
+enum { MaxCanIfaces = 3 };
+
 /**
  * Raw CAN frame, as passed to/from the CAN driver.
  */
@@ -137,6 +142,9 @@ public:
     /**
      * Non-blocking transmission.
      * If the frame wasn't transmitted upon TX deadline, the driver should discard it.
+     * Note that it is LIKELY that the library will want to send the frames that were passed into the select()
+     * method as the next ones to transmit, but it is NOT guaranteed. The library can replace those with new
+     * frames between the calls.
      * @return 1 = one frame transmitted, 0 = TX buffer full, negative for error.
      */
     virtual int16_t send(const CanFrame& frame, MonotonicTime tx_deadline, CanIOFlags flags) = 0;
@@ -202,12 +210,15 @@ public:
      * Iface masks will be modified by the driver to indicate which exactly interfaces are available for IO.
      * Bit position in the masks defines interface index.
      * Note that it is allowed to return from this method even if no requested events actually happened, or if
-     * there are events that were not requested by the lirary.
+     * there are events that were not requested by the library.
      * @param [in,out] inout_masks        Masks indicating which interfaces are needed/available for IO.
+     * @param [in]     pending_tx         Array of frames, per interface, that are likely to be transmitted next.
      * @param [in]     blocking_deadline  Zero means non-blocking operation.
      * @return Positive number of ready interfaces or negative error code.
      */
-    virtual int16_t select(CanSelectMasks& inout_masks, MonotonicTime blocking_deadline) = 0;
+    virtual int16_t select(CanSelectMasks& inout_masks,
+                           const CanFrame* (& pending_tx)[MaxCanIfaces],
+                           MonotonicTime blocking_deadline) = 0;
 };
 
 }

libuavcan/include/uavcan/transport/can_io.hpp

@@ -21,8 +21,6 @@
 namespace uavcan
 {
 
-enum { MaxCanIfaces = 3 };
-
 struct UAVCAN_EXPORT CanRxFrame : public CanFrame
 {
     MonotonicTime ts_mono;
@@ -108,7 +106,9 @@ public:
 
     Entry* peek();               // Modifier
     void remove(Entry*& entry);
+    const CanFrame* getTopPriorityPendingFrame() const;
 
+    /// The 'or equal' condition is necessary to avoid frame reordering.
     bool topPriorityHigherOrEqual(const CanFrame& rhs_frame) const;
 
     uint32_t getRejectedFrameCount() const { return rejected_frames_cnt_; }
@@ -154,7 +154,8 @@ class UAVCAN_EXPORT CanIOManager : Noncopyable
 
     int sendToIface(uint8_t iface_index, const CanFrame& frame, MonotonicTime tx_deadline, CanIOFlags flags);
     int sendFromTxQueue(uint8_t iface_index);
-    int callSelect(CanSelectMasks& inout_masks, MonotonicTime blocking_deadline);
+    int callSelect(CanSelectMasks& inout_masks, const CanFrame* (& pending_tx)[MaxCanIfaces],
+                   MonotonicTime blocking_deadline);
 
 public:
     CanIOManager(ICanDriver& driver, IPoolAllocator& allocator, ISystemClock& sysclock,