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

[Vincent Jardin]

SMBus 3.1 6.5.7 allows a Block Read byte count of 0, but the
interrupt-driven block-read state machine 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 of this i2c controller.

Accept count=0: NACK the in-flight dummy byte (TXAK) and set msg->len to
2 so i2c_imx_isr_read_continue() emits STOP via its normal last-byte
path. The dummy byte is discarded; block-read callers only consume
buf[0..count-1].

While here, return early on the I2C_SMBUS_BLOCK_MAX error path instead
of falling through and overwriting msg->len/msg->buf with the rejected
count byte.

The atomic path regressed earlier (v3.16) and is fixed separately; this
patch covers only the v6.13 state-machine rework.

Fixes: 5f5c2d4579ca ("i2c: imx: prevent rescheduling in non dma mode")
Cc: <stable at vger.kernel.org> # v6.13+
Signed-off-by: Vincent Jardin <vjardin at free.fr>
---
 drivers/i2c/busses/i2c-imx.c | 17 ++++++++++++++++-
 1 file changed, 16 insertions(+), 1 deletion(-)

diff --git a/drivers/i2c/busses/i2c-imx.c b/drivers/i2c/busses/i2c-imx.c
index 0cd4f5892591..8792cb5cb9a8 100644
--- a/drivers/i2c/busses/i2c-imx.c
+++ b/drivers/i2c/busses/i2c-imx.c
@@ -1061,11 +1061,26 @@ static inline enum imx_i2c_state i2c_imx_isr_read_continue(struct imx_i2c_struct
 static inline void i2c_imx_isr_read_block_data_len(struct imx_i2c_struct *i2c_imx)
 {
 	u8 len = imx_i2c_read_reg(i2c_imx, IMX_I2C_I2DR);
+	unsigned int temp;
 
-	if (len == 0 || len > I2C_SMBUS_BLOCK_MAX) {
+	if (len > I2C_SMBUS_BLOCK_MAX) {
 		i2c_imx->isr_result = -EPROTO;
 		i2c_imx->state = IMX_I2C_STATE_FAILED;
 		wake_up(&i2c_imx->queue);
+		return;
+	}
+
+	if (len == 0) {
+		/*
+		 * SMBus 3.1 6.5.7 "Block Write/Read": byte count can be 0
+		 */
+		temp = imx_i2c_read_reg(i2c_imx, IMX_I2C_I2CR);
+		temp |= I2CR_TXAK;
+		imx_i2c_write_reg(temp, i2c_imx, IMX_I2C_I2CR);
+
+		i2c_imx->msg->buf[i2c_imx->msg_buf_idx++] = 0;
+		i2c_imx->msg->len = 2;
+		return;
 	}
 	i2c_imx->msg->len += len;
 	i2c_imx->msg->buf[i2c_imx->msg_buf_idx++] = len;

-- 
2.43.0