tests: spi_loopback: Add testcases for same bufs

A common usage of the SPI API by in tree consumers is to have one
underlying buffer used to both RX to and TX from. Add some test cases
ensuring this works properly:
- A case to ensure that RX writes back properly the same data that was
  TX from the buf.
- A case simulating a very common paradigm which is to use one
  underlying buf but declare the spi buf sets as though the first word
  is a command from TX and the rest are data for RX.

Signed-off-by: Declan Snyder <[email protected]>

Author: decsny

tests/drivers/spi/spi_loopback/src/spi.c

@@ -360,6 +360,42 @@ ZTEST(spi_loopback, test_nop_nil_bufs)
 	/* nothing really to check here, check is done in spi_loopback_transceive */
 }
 
+/* test using the same buffer for RX and TX will write same data back */
+ZTEST(spi_loopback, test_spi_write_back)
+{
+	struct spi_dt_spec *spec = loopback_specs[spec_idx];
+	const struct spi_buf_set tx = spi_loopback_setup_xfer(rx_bufs_pool, 1,
+							      buffer_rx, BUF_SIZE);
+	const struct spi_buf_set rx = spi_loopback_setup_xfer(rx_bufs_pool, 1,
+							      buffer_rx, BUF_SIZE);
+
+	memcpy(buffer_rx, tx_data, sizeof(tx_data));
+
+	spi_loopback_transceive(spec, &tx, &rx);
+
+	spi_loopback_compare_bufs(tx_data, buffer_rx, BUF_SIZE,
+				  buffer_print_tx, buffer_print_rx);
+}
+
+/* similar to test_spi_write_back, simulates the real common case of 1 word command */
+ZTEST(spi_loopback, test_spi_same_buf_cmd)
+{
+	struct spi_dt_spec *spec = loopback_specs[spec_idx];
+	const struct spi_buf_set tx = spi_loopback_setup_xfer(rx_bufs_pool, 2,
+							      buffer_rx, 1,
+							      NULL, BUF_SIZE - 1);
+	const struct spi_buf_set rx = spi_loopback_setup_xfer(rx_bufs_pool, 1,
+							      NULL, BUF_SIZE - 1,
+							      buffer_rx+(BUF_SIZE - 1), 1);
+
+	memcpy(buffer_rx, tx_data, sizeof(tx_data));
+
+	spi_loopback_transceive(spec, &tx, &rx);
+
+	spi_loopback_compare_bufs(tx_data, buffer_rx, BUF_SIZE,
+				  buffer_print_tx, buffer_print_rx);
+}
+
 #if (CONFIG_SPI_ASYNC)
 static struct k_poll_signal async_sig = K_POLL_SIGNAL_INITIALIZER(async_sig);
 static struct k_poll_event async_evt =