[PATCH v2 1/2] i2c: imx: fix locked bus on SMBus block-read of 0 (atomic)

[Vincent Jardin]

SMBus 3.1 6.5.7 allows a Block Read byte count of 0, but the atomic
(polling) path rejects it as -EPROTO. Worse, it returns without a
NACK+STOP: the next receive cycle has already started, so the target
keeps holding SDA and the bus stays stuck until a power cycle for
this i2c controller.

Reading I2DR to obtain the count likewise arms the next byte on the
count > I2C_SMBUS_BLOCK_MAX path, which also returned -EPROTO directly
and left the bus held.

Handle both: NACK the in-flight dummy byte (TXAK) and extend msgs->len so
the existing last-byte handling emits STOP; the dummy byte is discarded.
A count of 0 is a valid empty block read; a count above
I2C_SMBUS_BLOCK_MAX is still reported as -EPROTO, but only after the bus
has been released.

The interrupt-driven path has the same flaw from a later commit and is
fixed separately, as it carries a different Fixes: tag and stable range.

Fixes: 8e8782c71595 ("i2c: imx: add SMBus block read support")
Cc: <stable at vger.kernel.org> # v3.16+
Signed-off-by: Vincent Jardin <vjardin at free.fr>
---
 drivers/i2c/busses/i2c-imx.c | 19 ++++++++++++++++---
 1 file changed, 16 insertions(+), 3 deletions(-)

diff --git a/drivers/i2c/busses/i2c-imx.c b/drivers/i2c/busses/i2c-imx.c
index a208fefd3c3b..14107e1ad413 100644
--- a/drivers/i2c/busses/i2c-imx.c
+++ b/drivers/i2c/busses/i2c-imx.c
@@ -1415,6 +1415,7 @@ static int i2c_imx_atomic_read(struct imx_i2c_struct *i2c_imx,
 	int i, result;
 	unsigned int temp;
 	int block_data = msgs->flags & I2C_M_RECV_LEN;
+	int block_err = 0;
 
 	result = i2c_imx_prepare_read(i2c_imx, msgs, false);
 	if (result)
@@ -1436,8 +1437,20 @@ static int i2c_imx_atomic_read(struct imx_i2c_struct *i2c_imx,
 		 */
 		if ((!i) && block_data) {
 			len = imx_i2c_read_reg(i2c_imx, IMX_I2C_I2DR);
-			if ((len == 0) || (len > I2C_SMBUS_BLOCK_MAX))
-				return -EPROTO;
+			if ((len == 0) || (len > I2C_SMBUS_BLOCK_MAX)) {
+				/*
+				 * SMBus 3.1 6.5.7: support count byte of 0.
+				 * I2C_SMBUS_BLOCK_MAX case should not hold the SDA either.
+				 */
+				if (len > I2C_SMBUS_BLOCK_MAX)
+					block_err = -EPROTO;
+				temp = imx_i2c_read_reg(i2c_imx, IMX_I2C_I2CR);
+				temp |= I2CR_TXAK;
+				imx_i2c_write_reg(temp, i2c_imx, IMX_I2C_I2CR);
+				msgs->buf[0] = 0;
+				msgs->len = 2;
+				continue;
+			}
 			dev_dbg(&i2c_imx->adapter.dev,
 				"<%s> read length: 0x%X\n",
 				__func__, len);
@@ -1485,7 +1498,7 @@ static int i2c_imx_atomic_read(struct imx_i2c_struct *i2c_imx,
 			"<%s> read byte: B%d=0x%X\n",
 			__func__, i, msgs->buf[i]);
 	}
-	return 0;
+	return block_err;
 }
 
 static int i2c_imx_read(struct imx_i2c_struct *i2c_imx, struct i2c_msg *msgs,

-- 
2.43.0