[PATCH v2] i2c: designware: Only check the first byte for SMBus block read length

[Sultan Alsawaf]

From: Sultan Alsawaf <sultan@xxxxxxxxxxxxxxx>

SMBus block reads can be broken because the read function will just skip
over bytes it doesn't like until reaching a byte that conforms to the
length restrictions for block reads. This is problematic when it isn't
known if the incoming payload is indeed a conforming block read.

According to the SMBus specification, block reads will only send the
payload length in the first byte, so we can fix this by only considering
the first byte in a sequence for block read length purposes.

Fixes: c3ae106050b9 ("i2c: designware: Implement support for SMBus block read and write")
Signed-off-by: Sultan Alsawaf <sultan@xxxxxxxxxxxxxxx>
---
 drivers/i2c/busses/i2c-designware-master.c | 16 +++++++---------
 1 file changed, 7 insertions(+), 9 deletions(-)

diff --git a/drivers/i2c/busses/i2c-designware-master.c b/drivers/i2c/busses/i2c-designware-master.c
index d6425ad6e6a3..d22271438869 100644
--- a/drivers/i2c/busses/i2c-designware-master.c
+++ b/drivers/i2c/busses/i2c-designware-master.c
@@ -391,14 +391,10 @@ i2c_dw_recv_len(struct dw_i2c_dev *dev, u8 len)
 	struct i2c_msg *msgs = dev->msgs;
 	u32 flags = msgs[dev->msg_read_idx].flags;
 
-	/*
-	 * Adjust the buffer length and mask the flag
-	 * after receiving the first byte.
-	 */
+	/* Adjust the buffer length */
 	len += (flags & I2C_CLIENT_PEC) ? 2 : 1;
 	dev->tx_buf_len = len - min_t(u8, len, dev->rx_outstanding);
 	msgs[dev->msg_read_idx].len = len;
-	msgs[dev->msg_read_idx].flags &= ~I2C_M_RECV_LEN;
 
 	return len;
 }
@@ -430,10 +426,12 @@ i2c_dw_read(struct dw_i2c_dev *dev)
 			u32 flags = msgs[dev->msg_read_idx].flags;
 
 			regmap_read(dev->map, DW_IC_DATA_CMD, &tmp);
-			/* Ensure length byte is a valid value */
-			if (flags & I2C_M_RECV_LEN &&
-			    tmp <= I2C_SMBUS_BLOCK_MAX && tmp > 0) {
-				len = i2c_dw_recv_len(dev, tmp);
+			if (flags & I2C_M_RECV_LEN) {
+				/* Ensure length byte is a valid value */
+				if (tmp <= I2C_SMBUS_BLOCK_MAX && tmp > 0)
+					len = i2c_dw_recv_len(dev, tmp);
+				/* Mask the flag after receiving the first byte */
+				msgs[dev->msg_read_idx].flags &= ~I2C_M_RECV_LEN;
 			}
 			*buf++ = tmp;
 			dev->rx_outstanding--;
-- 
2.27.0