[PATCH v7 2/3] i2c: xiic: defer RX_FULL until all trailing bytes are in FIFO

[Abdurrahman Hussain <abdurrahman@nexthop.ai>]

For the normal path of xiic_smbus_block_read_setup() -- the trailing
bytes all fit in one Rx FIFO fill -- RFD was programmed two below the
byte count, which fires the RX_FULL interrupt while the last byte is
still in flight. xiic_read_rx() then lands in its bytes_rem == 1 branch
and sets NACK on a byte still on the wire, truncating the read.

Without PEC this is harmless: the truncated byte is the dummy one the
caller never looks at. With PEC enabled it is the PEC byte itself, and
i2c_smbus_check_pec() fails the transfer with -EBADMSG.

Raise the threshold by one so RX_FULL fires only once every remaining
byte is already buffered. That routes the drain through
xiic_read_rx()'s bytes_rem == 0 path, which reads everything out and
emits the stop cleanly. The only change for the non-PEC case is that
the controller waits one extra byte-time before servicing the
interrupt.

rfd_set stays inside the 4 bits of XIIC_RFD_REG_OFFSET: this branch is
only reached when rxmsg_len + pec_len <= IIC_RX_FIFO_DEPTH, so the
value is at most IIC_RX_FIFO_DEPTH - 1.

Fixes: e4c1ff772e1a ("i2c: xiic: Add smbus_block_read functionality")
Cc: stable@vger.kernel.org
Acked-by: Michal Simek <michal.simek@amd.com>
Signed-off-by: Abdurrahman Hussain <abdurrahman@nexthop.ai>
---
 drivers/i2c/busses/i2c-xiic.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/drivers/i2c/busses/i2c-xiic.c b/drivers/i2c/busses/i2c-xiic.c
index 0777de45bdf4..5cd737c7608f 100644
--- a/drivers/i2c/busses/i2c-xiic.c
+++ b/drivers/i2c/busses/i2c-xiic.c
@@ -569,11 +569,11 @@ static void xiic_smbus_block_read_setup(struct xiic_i2c *i2c)
 			i2c->smbus_actual_len = 1 + rxmsg_len + pec_len;
 		} else {
 			/*
-			 * All trailing bytes fit in the Rx FIFO. The widened
-			 * condition above guarantees rxmsg_len + pec_len >= 2,
-			 * so this cannot underflow.
+			 * All trailing bytes fit in the Rx FIFO. Defer RX_FULL
+			 * until every one of them is buffered, so the drain
+			 * takes xiic_read_rx()'s bytes_rem == 0 path.
 			 */
-			rfd_set = rxmsg_len + pec_len - 2;
+			rfd_set = rxmsg_len + pec_len - 1;
 			i2c->rx_msg->len = rxmsg_len + 1 + pec_len;
 		}
 		xiic_setreg8(i2c, XIIC_RFD_REG_OFFSET, rfd_set);

-- 
2.54.0