i2c: imx: fix locked bus on SMBus block-read of 0 (atomic)

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")
Signed-off-by: Vincent Jardin <vjardin@free.fr>
Cc: <stable@vger.kernel.org> # v3.16+
Acked-by: Oleksij Rempel <o.rempel@pengutronix.de>
Acked-by: Carlos Song <carlos.song@nxp.com>
Reviewed-by: Stefan Eichenberger <eichest@gmail.com>
Signed-off-by: Andi Shyti <andi.shyti@kernel.org>
Link: https://lore.kernel.org/r/20260713-for-upstream-i2c-lx2160-fix-v1-v3-1-073ac9e103a5@free.fr

diff --git a/drivers/i2c/busses/i2c-imx.c b/drivers/i2c/busses/i2c-imx.c
index 28313d0fad37e161f27991d7449bf3f5a43ffb05..cfd1e63359e75c38970c6851aacb642a0626c17b 100644 (file)
--- 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,