diff --git a/include/spi.h b/include/spi.h
index 8cfd934..6483e9b 100644
--- a/include/spi.h
+++ b/include/spi.h
@@ -4,8 +4,8 @@ struct spi_dev;
 
 struct spi_ops {
 	int (* set_cs_level)(struct spi_dev *dev, int level);
-	int (* tx_rx)(struct spi_dev *dev, void *rx_buf, const void *tx_buf,
-		size_t len);
+	int (* tx_rx)(struct spi_dev *dev, void *rx_buf, size_t rx_len,
+		const void *tx_buf, size_t tx_len);
 };
 
 struct spi_dev {
@@ -16,5 +16,5 @@ struct spi_dev {
 struct spi_dev *spi_probe(void);
 void spi_release(struct spi_dev *dev);
 int spi_set_cs_level(struct spi_dev *dev, int level);
-int spi_tx_rx(struct spi_dev *dev, void *rx_buf, const void *tx_buf,
-	size_t len);
+int spi_tx_rx(struct spi_dev *dev, void *rx_buf, size_t rx_len,
+	const void *tx_buf, size_t tx_len);
diff --git a/source/core/spi.c b/source/core/spi.c
index b9e5816..a949630 100644
--- a/source/core/spi.c
+++ b/source/core/spi.c
@@ -11,11 +11,11 @@ int spi_set_cs_level(struct spi_dev *dev, int level)
 	return dev->ops->set_cs_level(dev, level);
 }
 
-int spi_tx_rx(struct spi_dev *dev, void *rx_buf, const void *tx_buf,
-	size_t len)
+int spi_tx_rx(struct spi_dev *dev, void *rx_buf, size_t rx_len,
+	const void *tx_buf, size_t tx_len)
 {
 	if (!dev)
 		return -1;
 
-	return dev->ops->tx_rx(dev, rx_buf, tx_buf, len);
+	return dev->ops->tx_rx(dev, rx_buf, rx_len, tx_buf, tx_len);
 }
diff --git a/source/drivers/spi_flash.c b/source/drivers/spi_flash.c
index 2e100ab..78471c6 100644
--- a/source/drivers/spi_flash.c
+++ b/source/drivers/spi_flash.c
@@ -36,9 +36,7 @@ static void spi_flash_write_enable(struct flash_dev *dev)
 	char cmd[1] = { SPI_FLASH_WRITE_ENABLE };
 	struct spi_dev *spi_dev = dev->priv;
 
-	spi_set_cs_level(spi_dev, 0);
-	spi_tx_rx(spi_dev, NULL, cmd, sizeof cmd);
-	spi_set_cs_level(spi_dev, 1);
+	spi_tx_rx(spi_dev, NULL, 0, cmd, sizeof cmd);
 }
 
 static void spi_flash_write_disable(struct flash_dev *dev)
@@ -46,23 +44,19 @@ static void spi_flash_write_disable(struct flash_dev *dev)
 	char cmd[1] = { SPI_FLASH_WRITE_DISABLE };
 	struct spi_dev *spi_dev = dev->priv;
 
-	spi_set_cs_level(spi_dev, 0);
-	spi_tx_rx(spi_dev, NULL, cmd, sizeof cmd);
-	spi_set_cs_level(spi_dev, 1);
+	spi_tx_rx(spi_dev, NULL, 0, cmd, sizeof cmd);
 }
 
 static size_t spi_flash_get_jedec_id(struct flash_dev *dev, uint8_t *jedec_id,
 	size_t len)
 {
-	uint8_t cmd[5] = { SPI_FLASH_JEDEC_ID, 0, 0, 0, 0 };
-	uint8_t buf[5];
+	uint8_t cmd[1] = { SPI_FLASH_JEDEC_ID };
+	uint8_t buf[6];
 	struct spi_dev *spi_dev = dev->priv;
 
-	spi_set_cs_level(spi_dev, 0);
-	spi_tx_rx(spi_dev, buf, cmd, sizeof cmd);
-	spi_set_cs_level(spi_dev, 1);
+	spi_tx_rx(spi_dev, buf, sizeof buf, cmd, sizeof cmd);
 
-	memcpy(jedec_id, buf + 1, min(4, len));
+	memcpy(jedec_id, buf + 1, min(sizeof buf, len));
 
 	return len;
 }
@@ -87,8 +81,7 @@ static size_t spi_flash_read(struct flash_dev *dev, uint32_t addr, void *data,
 	spi_flash_addr(cmd, addr);
 
 	spi_set_cs_level(spi_dev, 0);
-	spi_tx_rx(spi_dev, NULL, cmd, sizeof cmd);
-	spi_tx_rx(spi_dev, data, NULL, len);
+	spi_tx_rx(spi_dev, data, len, cmd, sizeof cmd);
 	spi_set_cs_level(spi_dev, 1);
 
 	return len;
@@ -97,15 +90,17 @@ static size_t spi_flash_read(struct flash_dev *dev, uint32_t addr, void *data,
 static size_t spi_flash_write(struct flash_dev *dev, uint32_t addr,
 	const void *data, size_t len)
 {
-	char cmd[4] = { SPI_FLASH_PAGE_PROGRAM };
+	char cmd[4 + len];
 	struct spi_dev *spi_dev = dev->priv;
 
 	spi_flash_write_enable(dev);
+
+	cmd[0] = SPI_FLASH_PAGE_PROGRAM;
 	spi_flash_addr(cmd, addr);
+	memcpy(cmd + 4, data, len);
 
 	spi_set_cs_level(spi_dev, 0);
-	spi_tx_rx(spi_dev, NULL, cmd, sizeof cmd);
-	spi_tx_rx(spi_dev, NULL, data, len);
+	spi_tx_rx(spi_dev, NULL, 0, cmd, sizeof cmd);
 	spi_set_cs_level(spi_dev, 1);
 
 	spi_flash_write_disable(dev);
@@ -124,7 +119,7 @@ static int spi_flash_erase(struct flash_dev *dev, uint32_t addr)
 	spi_flash_addr(cmd, addr << dev->log2_block_size);
 
 	spi_set_cs_level(spi_dev, 0);
-	spi_tx_rx(spi_dev, NULL, cmd, sizeof cmd);
+	spi_tx_rx(spi_dev, NULL, 0, cmd, sizeof cmd);
 	spi_set_cs_level(spi_dev, 1);
 
 	spi_flash_write_disable(dev);
diff --git a/source/drivers/stm32f0_spi.c b/source/drivers/stm32f0_spi.c
index cd987d4..4ee2806 100644
--- a/source/drivers/stm32f0_spi.c
+++ b/source/drivers/stm32f0_spi.c
@@ -10,8 +10,8 @@
 #include <spi.h>
 
 static int stm32f0_spi_set_cs_level(struct spi_dev *dev, int level);
-static int stm32f0_spi_tx_rx(struct spi_dev *dev, void *rx_buf,
-	const void *tx_buf, size_t len);
+static int stm32f0_spi_tx_rx(struct spi_dev *dev, void *rx_buf, size_t rx_len,
+	const void *tx_buf, size_t tx_len);
 
 static struct spi_ops stm32f0_spi_ops = {
 	.set_cs_level = stm32f0_spi_set_cs_level,
@@ -80,20 +80,26 @@ static int stm32f0_spi_set_cs_level(struct spi_dev *dev, int level)
 	return 0;
 }
 
-static int stm32f0_spi_tx_rx(struct spi_dev *dev, void *rx_buf,
-	const void *tx_buf, size_t len)
+static int stm32f0_spi_tx_rx(struct spi_dev *dev, void *rx_buf, size_t rx_len,
+	const void *tx_buf, size_t tx_len)
 {
 	char *rx = rx_buf;
 	const char *tx = tx_buf;
 	size_t i;
 
-	for (i = 0; i < len; ++i) {
+	gpio_clear(GPIOA, GPIO4);
+
+	for (i = 0; i < tx_len; ++i) {
 		spi_send8(dev->dev_id, tx ? *tx++ : 0);
+	}
 
-		if (rx) {
+	if (rx_buf) {
+		for (i = 0; i < rx_len; ++i) {
 			*rx++ = spi_read8(dev->dev_id);
 		}
 	}
 
+	gpio_set(GPIOA, GPIO4);
+
 	return 0;
 }