mtd: m25p80: return amount of data transferred or error in read/write

Add checking of SPI transfer errors and return them from read/write
functions. Also return the amount of data transferred.
Signed-off-by: Michal Suchanek <hramrach@gmail.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Acked-by: Michal Suchanek <hramrach@gmail.com>
Tested-by: Michal Suchanek <hramrach@gmail.com>

drivers/mtd/devices/m25p80.c

@@ -81,6 +81,7 @@ static ssize_t m25p80_write(struct spi_nor *nor, loff_t to, size_t len,
 	struct spi_transfer t[2] = {};
 	struct spi_message m;
 	int cmd_sz = m25p_cmdsz(nor);
+	ssize_t ret;
 
 	spi_message_init(&m);
 
@@ -98,10 +99,15 @@ static ssize_t m25p80_write(struct spi_nor *nor, loff_t to, size_t len,
 	t[1].len = len;
 	spi_message_add_tail(&t[1], &m);
 
-	spi_sync(spi, &m);
+	ret = spi_sync(spi, &m);
+	if (ret)
+		return ret;
 
-	*retlen += m.actual_length - cmd_sz;
-	return 0;
+	ret = m.actual_length - cmd_sz;
+	if (ret < 0)
+		return -EIO;
+	*retlen += ret;
+	return ret;
 }
 
 static inline unsigned int m25p80_rx_nbits(struct spi_nor *nor)
@@ -128,13 +134,13 @@ static ssize_t m25p80_read(struct spi_nor *nor, loff_t from, size_t len,
 	struct spi_transfer t[2];
 	struct spi_message m;
 	unsigned int dummy = nor->read_dummy;
+	ssize_t ret;
 
 	/* convert the dummy cycles to the number of bytes */
 	dummy /= 8;
 
 	if (spi_flash_read_supported(spi)) {
 		struct spi_flash_read_message msg;
-		int ret;
 
 		memset(&msg, 0, sizeof(msg));
 
@@ -151,7 +157,9 @@ static ssize_t m25p80_read(struct spi_nor *nor, loff_t from, size_t len,
 
 		ret = spi_flash_read(spi, &msg);
 		*retlen = msg.retlen;
-		return ret;
+		if (ret < 0)
+			return ret;
+		return msg.retlen;
 	}
 
 	spi_message_init(&m);
@@ -169,10 +177,15 @@ static ssize_t m25p80_read(struct spi_nor *nor, loff_t from, size_t len,
 	t[1].len = len;
 	spi_message_add_tail(&t[1], &m);
 
-	spi_sync(spi, &m);
+	ret = spi_sync(spi, &m);
+	if (ret)
+		return ret;
 
-	*retlen = m.actual_length - m25p_cmdsz(nor) - dummy;
-	return 0;
+	ret = m.actual_length - m25p_cmdsz(nor) - dummy;
+	if (ret < 0)
+		return -EIO;
+	*retlen += ret;
+	return ret;
 }
 
 /*