spi: Add SPI transfer function that pass in RX/TX

When using SPI, I prefer to not have to do malloc/free functions for each
transfer, so why not have a transfer function that you can pass in both
buffers.  With my ILI9341 TFT display code that gave some perf wins, also more
of a win, you can pass in NULL for recv buffer and the underlying device driver
does not have to copy the data.

Signed-off-by: Kurt Eckhardt <kurte@rockisland.com>
Signed-off-by: Brendan Le Foll <brendan.le.foll@intel.com>

api/mraa/spi.h

@@ -105,6 +105,17 @@ uint8_t mraa_spi_write(mraa_spi_context dev, uint8_t data);
  */
 uint8_t* mraa_spi_write_buf(mraa_spi_context dev, uint8_t* data, int length);
 
+/** Transfer Buffer of bytes to the SPI device. Both send and recv buffers
+ * are passed in
+ *
+ * @param dev The Spi context
+ * @param data to send
+ * @param rxbuf buffer to recv data back, may be NULL
+ * @param length elements within buffer, Max 4096
+ * @return Result of operation
+ */
+mraa_result_t mraa_spi_transfer_buf(mraa_spi_context dev, uint8_t* data, uint8_t* rxbuf, int length);
+
 /**
  * Change the SPI lsb mode
  *

api/mraa/spi.hpp

@@ -84,7 +84,8 @@ class Spi {
             return (char) mraa_spi_write(m_spi, (uint8_t) data);
         }
         /**
-         * Write buffer of bytes to SPI device
+         * Write buffer of bytes to SPI device The pointer return has to be
+         * free'd by the caller.
          *
          * @param data buffer to send
          * @param length size of buffer to send
@@ -93,6 +94,18 @@ class Spi {
         char* write(char* data, size_t length) {
             return (char*) mraa_spi_write_buf(m_spi, (uint8_t *) data, (int) length);
         }
+        /**
+         * Transfer data to and from SPI device Receive pointer may be null if return
+         * data is not needed.
+         *
+         * @param data buffer to send
+         * @param rxBuf buffer to optionally receive data from spi device
+         * @param length size of buffer to send
+         * @return Result of operation
+         */
+        mraa_result_t transfer(char* data, char* rxBuf, size_t length) {
+            return mraa_spi_transfer_buf(m_spi, (uint8_t *) data, (uint8_t *)rxBuf, (int) length);
+        }
         /**
          * Change the SPI lsb mode
          *

examples/c++/Spi-pot.cpp

@@ -49,6 +49,7 @@ int main ()
     spi = new mraa::Spi(0);
 
     char data[] = {0x00, 100};
+    char rxBuf[2];
     char *recv;
     while (running == 0) {
         int i;
@@ -56,14 +57,18 @@ int main ()
             data[1] = i;
             recv = spi->write(data, 2);
             printf("Writing -%i",i);
+            if (recv) {
                 printf("RECIVED-%i-%i\n",recv[0],recv[1]);
+                free(recv);
+            }
             usleep(100000);
         }
         for (i = 130; i > 90; i--) {
             data[1] = i;
-            recv = spi->write(data, 2);
+            if (spi->transfer(data, rxBuf, 2) == MRAA_SUCCESS) {
                 printf("Writing -%i",i);
-            printf("RECIVED-%i-%i\n",recv[0],recv[1]);
+                printf("RECIVED-%i-%i\n",rxBuf[0],rxBuf[1]);
+            }
             usleep(100000);
         }
 

src/spi/spi.c

@@ -193,6 +193,25 @@ mraa_spi_write_buf(mraa_spi_context dev, uint8_t* data, int length)
     return recv;
 }
 
+mraa_result_t
+mraa_spi_transfer_buf(mraa_spi_context dev, uint8_t* data, uint8_t* rxbuf, int length)
+{
+    struct spi_ioc_transfer msg;
+    memset(&msg, 0, sizeof(msg));
+
+    msg.tx_buf = (unsigned long) data;
+    msg.rx_buf = (unsigned long) rxbuf;
+    msg.speed_hz = dev->clock;
+    msg.bits_per_word = dev->bpw;
+    msg.delay_usecs = 0;
+    msg.len = length;
+    if (ioctl(dev->devfd, SPI_IOC_MESSAGE(1), &msg) < 0) {
+        syslog(LOG_ERR, "Failed to perform dev transfer");
+        return MRAA_ERROR_INVALID_RESOURCE;
+    }
+    return MRAA_SUCCESS;
+}
+
 mraa_result_t
 mraa_spi_stop(mraa_spi_context dev)
 {