drivers: flash: spi_nor: optimize SPI buffer usage

This driver was providing SPI buffers for both TX and RX on the data
payload portion of read transfers, even though the TX buffer is not
meaningful in these cases. As well as being less efficient, this also
caused likely uninitialized data to be transferred to the device, which
is possibly problematic.

Update to not include the TX buffer for the read data payload SPI
transfer, so that the SPI driver can generate dummy TX data internally.

Signed-off-by: Robert Hancock <robert.hancock@calian.com>

Author: robhancocksed

drivers/flash/spi_nor.c

@@ -381,8 +381,8 @@ static int spi_nor_access(const struct device *const dev,
 	struct spi_nor_data *const driver_data = dev->data;
 	bool is_addressed = (access & NOR_ACCESS_ADDRESSED) != 0U;
 	bool is_write = (access & NOR_ACCESS_WRITE) != 0U;
-	uint8_t buf[5] = { 0 };
-	struct spi_buf spi_buf[2] = {
+	uint8_t buf[5] = {opcode};
+	struct spi_buf spi_buf_tx[2] = {
 		{
 			.buf = buf,
 			.len = 1,
@@ -392,8 +392,17 @@ static int spi_nor_access(const struct device *const dev,
 			.len = length
 		}
 	};
+	struct spi_buf spi_buf_rx[2] = {
+		{
+			.buf = NULL,
+			.len = 1,
+		},
+		{
+			.buf = data,
+			.len = length
+		}
+	};
 
-	buf[0] = opcode;
 	if (is_addressed) {
 		bool access_24bit = (access & NOR_ACCESS_24BIT_ADDR) != 0;
 		bool access_32bit = (access & NOR_ACCESS_32BIT_ADDR) != 0;
@@ -409,20 +418,22 @@ static int spi_nor_access(const struct device *const dev,
 
 		if (use_32bit) {
 			memcpy(&buf[1], &addr32.u8[0], 4);
-			spi_buf[0].len += 4;
+			spi_buf_tx[0].len += 4;
+			spi_buf_rx[0].len += 4;
 		} else {
 			memcpy(&buf[1], &addr32.u8[1], 3);
-			spi_buf[0].len += 3;
+			spi_buf_tx[0].len += 3;
+			spi_buf_rx[0].len += 3;
 		}
 	};
 
 	const struct spi_buf_set tx_set = {
-		.buffers = spi_buf,
-		.count = (length != 0) ? 2 : 1,
+		.buffers = spi_buf_tx,
+		.count = (is_write && length != 0) ? 2 : 1,
 	};
 
 	const struct spi_buf_set rx_set = {
-		.buffers = spi_buf,
+		.buffers = spi_buf_rx,
 		.count = 2,
 	};