STM32: SPI 3 wires mode not supported in SPI slave

This patch handles the case of SPI slave mode without MISO (NC).
In case MISO is not connected, we consider that SPI will be configured in
3 wires mode (CLK / MOSI / CS, but no MISO). In this case, the MOSI line
is bi-directional : SPI_DIRECTION_1LINE.

But as this is not supported yet in slave mode, we force it to
SPI_DIRECTION_2LINES. In this case slave SPI will receive data on MOSI
but nothing will be sent back to master as MISO is not connected.

Author: LMESTM

targets/TARGET_STM/stm_spi_api.c

@@ -29,6 +29,7 @@
  */
 #include "mbed_assert.h"
 #include "mbed_error.h"
+#include "mbed_debug.h"
 #include "spi_api.h"
 
 #if DEVICE_SPI
@@ -286,6 +287,16 @@ void spi_format(spi_t *obj, int bits, int mode, int slave)
 
     handle->Init.Mode = (slave) ? SPI_MODE_SLAVE : SPI_MODE_MASTER;
 
+    if (slave && (handle->Init.Direction == SPI_DIRECTION_1LINE)) {
+        /*  SPI slave implemtation in MBED does not support the 3 wires SPI.
+         *  (e.g. when MISO is not connected). So we're forcing slave in
+         *  2LINES mode. As MISO is not connected, slave will only read
+         *  from master, and cannot write to it. Inform user.
+         */
+        debug("3 wires SPI slave not supported - slave will only read\r\n");
+        handle->Init.Direction = SPI_DIRECTION_2LINES;
+    }
+
     init_spi(obj);
 }