tests: spi_loopback: Fix same_buf_cmd test

The tests was written wrong. It was meant to test using same spi bufs
for both rx and tx, as in tree many sensor and other spi device drivers
use this paradigm. But the 2nd buf setup call was overwriting the first.
Fix by not using the helper function for this case.

And for the write back test, test using same spi_buf_set.

Signed-off-by: Declan Snyder <declan.snyder@nxp.com>

Author: decsny

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

@@ -599,18 +599,17 @@ 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 */
+/* test using the same buffer set 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);
+
+	struct spi_buf buf = {.buf = buffer_rx, .len = BUF_SIZE};
+	struct spi_buf_set set = {.buffers = &buf, .count = 1};
 
 	memcpy(buffer_rx, tx_data, sizeof(tx_data));
 
-	spi_loopback_transceive(spec, &tx, &rx);
+	spi_loopback_transceive(spec, &set, &set);
 
 	spi_loopback_compare_bufs(tx_data, buffer_rx, BUF_SIZE,
 				  buffer_print_tx, buffer_print_rx);
@@ -620,19 +619,25 @@ ZTEST(spi_loopback, test_spi_write_back)
 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));
+	struct spi_buf buf[2] = {
+		{.buf = buffer_rx, .len = 1},
+		{.buf = buffer_rx+1, .len = BUF_SIZE - 1}
+	};
+
+	const struct spi_buf_set tx = {.buffers = buf, .count = 1};
+	const struct spi_buf_set rx = {.buffers = buf, .count = 2};
+
+	memcpy(buffer_rx, tx_data, BUF_SIZE);
 
 	spi_loopback_transceive(spec, &tx, &rx);
 
-	spi_loopback_compare_bufs(tx_data, buffer_rx, BUF_SIZE,
+	spi_loopback_compare_bufs(tx_data, buffer_rx, 1,
 				  buffer_print_tx, buffer_print_rx);
+
+	char zeros[BUF_SIZE - 1] = {0};
+
+	zassert_ok(memcmp(buffer_rx+1, zeros, BUF_SIZE - 1));
 }