HAL_ChibiOS: enable checking for DMA contention

libraries/AP_HAL_ChibiOS/shared_dma.cpp

@@ -105,6 +105,12 @@ bool Shared_DMA::lock_nonblock(void)
 {
     if (stream_id1 != SHARED_DMA_NONE) {
         if (chBSemWaitTimeout(&locks[stream_id1].semaphore, 1) != MSG_OK) {
+            chSysDisable();
+            if (locks[stream_id1].obj != nullptr && locks[stream_id1].obj != this) {
+                locks[stream_id1].obj->contention = true;
+            }
+            chSysEnable();
+            contention = true;
             return false;
         }
     }
@@ -113,6 +119,12 @@ bool Shared_DMA::lock_nonblock(void)
             if (stream_id1 != SHARED_DMA_NONE) {
                 chBSemSignal(&locks[stream_id1].semaphore);
             }
+            chSysDisable();
+            if (locks[stream_id2].obj != nullptr && locks[stream_id2].obj != this) {
+                locks[stream_id2].obj->contention = true;
+            }
+            chSysEnable();
+            contention = true;
             return false;
         }
     }

libraries/AP_HAL_ChibiOS/shared_dma.h

@@ -58,6 +58,10 @@ public:
     //should be called inside the destructor of Shared DMA participants
     void unregister(void);
 
+    // return true if this DMA channel is being actively contended for
+    // by multiple drivers
+    bool has_contention(void) const { return contention; }
+    
     // lock all shared DMA channels. Used on reboot
     static void lock_all(void);
     
@@ -68,6 +72,10 @@ private:
     uint8_t stream_id2;
     bool have_lock;
 
+    // we set the contention flag if two drivers are fighting over a DMA channel.
+    // the UART driver uses this to change its max transmit size to reduce latency
+    bool contention;
+
     // core of lock call, after semaphores gained
     void lock_core(void);