[PATCH v5 3/3] i2c: xiic: don't clobber msg->len to signal block-read completion

[Abdurrahman Hussain <abdurrahman@nexthop.ai>]

At the end of a SMBus block read the BNB handler force-set
tx_msg->len = 1 to push xiic_tx_space() to zero so the STATE_DONE
branch would fire. Two problems:

1. tx_msg and rx_msg alias the same i2c_msg struct during a receive
   (see xiic_start_recv), so overwriting tx_msg->len also changes
   rx_msg->len. The i2c core's i2c_smbus_check_pec() then reads the
   PEC from the wrong offset -- buf[0] instead of buf[rxmsg_len + 1]
   -- and either mis-validates or returns -EBADMSG.

2. xiic_start_recv sets tx_pos = msg->len (typically 2 when PEC is
   enabled). xiic_tx_space() is unsigned msg->len - tx_pos, so
   setting msg->len = 1 with tx_pos = 2 underflows to 0xFFFFFFFF and
   xiic_tx_space() never compares equal to 0 -- the STATE_DONE check
   falls through to STATE_ERROR, giving -EIO.

Instead, advance tx_pos up to msg->len. That drives tx_space to 0
without touching msg->len, preserving the buffer length that
xiic_smbus_block_read_setup() already grew to cover the length byte,
the payload and the optional PEC byte.

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 | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/drivers/i2c/busses/i2c-xiic.c b/drivers/i2c/busses/i2c-xiic.c
index 15fd17b703bc..d5e7b089a2b8 100644
--- a/drivers/i2c/busses/i2c-xiic.c
+++ b/drivers/i2c/busses/i2c-xiic.c
@@ -879,8 +879,11 @@ static irqreturn_t xiic_process(int irq, void *dev_id)
 
 		if (i2c->tx_msg && i2c->smbus_block_read) {
 			i2c->smbus_block_read = false;
-			/* Set requested message len=1 to indicate STATE_DONE */
-			i2c->tx_msg->len = 1;
+			/*
+			 * Drive xiic_tx_space() to 0 to signal STATE_DONE
+			 * without truncating the rx_msg length.
+			 */
+			i2c->tx_pos = i2c->tx_msg->len;
 		}
 
 		if (!i2c->tx_msg)

-- 
2.54.0