HAL_PX4: avoid bounce buffers for SPI when possible

libraries/AP_HAL_PX4/SPIDevice.cpp

@@ -146,7 +146,7 @@ bool SPIDevice::set_speed(AP_HAL::Device::Speed speed)
 /*
   low level transfer function
  */
-void SPIDevice::do_transfer(uint8_t *send, uint8_t *recv, uint32_t len)
+void SPIDevice::do_transfer(const uint8_t *send, uint8_t *recv, uint32_t len)
 {
     /*
       to accomodate the method in PX4 drivers of using interrupt
@@ -187,6 +187,11 @@ void SPIDevice::do_transfer(uint8_t *send, uint8_t *recv, uint32_t len)
 bool SPIDevice::transfer(const uint8_t *send, uint32_t send_len,
                          uint8_t *recv, uint32_t recv_len)
 {
+    if (send_len == recv_len && send == recv) {
+        // simplest cases, needed for DMA
+        do_transfer(send, recv, recv_len);
+        return true;
+    }
     uint8_t buf[send_len+recv_len];
     if (send_len > 0) {
         memcpy(buf, send, send_len);

libraries/AP_HAL_PX4/SPIDevice.h

@@ -62,7 +62,7 @@ public:
     bool set_speed(AP_HAL::Device::Speed speed) override;
 
     // low level transfer function
-    void do_transfer(uint8_t *send, uint8_t *recv, uint32_t len);
+    void do_transfer(const uint8_t *send, uint8_t *recv, uint32_t len);
     
     /* See AP_HAL::Device::transfer() */
     bool transfer(const uint8_t *send, uint32_t send_len,