[PATCH 1/3] i2c: xiic: preserve PEC byte length in SMBus block read setup

[Abdurrahman Hussain via B4 Relay <devnull+abdurrahman.nexthop.ai@kernel.org>]

From: Abdurrahman Hussain <abdurrahman@nexthop.ai>

xiic_smbus_block_read_setup() overwrites rx_msg->len based on the
announced block length byte, but the i2c core appends an extra byte to
msg->len when PEC is enabled on the client (see
__i2c_smbus_xfer_emulated in i2c-core-smbus.c). Before this handler
runs, rx_msg->len is therefore 1 + pec_len, where pec_len is 0 or 1.

Overwriting rx_msg->len to rxmsg_len + 1 drops that PEC byte from the
caller's buffer: the PEC byte is never drained from the FIFO and the
core's i2c_smbus_check_pec() reads the last payload byte instead of
the actual PEC, returning -EBADMSG even on clean transfers. Capture
pec_len before the overwrite and add it back when recomputing
rx_msg->len.

This is a pure bug fix; non-PEC behaviour is unchanged (pec_len == 0).
Tested on a Xilinx AXI IIC FPGA block driving an adm1266 PMBus
blackbox read -- with PEC enabled the full 64-byte record transfers
cleanly after this change.

Signed-off-by: Abdurrahman Hussain <abdurrahman@nexthop.ai>
---
 drivers/i2c/busses/i2c-xiic.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/drivers/i2c/busses/i2c-xiic.c b/drivers/i2c/busses/i2c-xiic.c
index 3e7735e1dae0..959a47b645a5 100644
--- a/drivers/i2c/busses/i2c-xiic.c
+++ b/drivers/i2c/busses/i2c-xiic.c
@@ -539,6 +539,8 @@ static void xiic_smbus_block_read_setup(struct xiic_i2c *i2c)
 
 	/* Check if received length is valid */
 	if (rxmsg_len <= I2C_SMBUS_BLOCK_MAX) {
+		unsigned int pec_len = i2c->rx_msg->len - 1;
+
 		/* Set Receive fifo depth */
 		if (rxmsg_len > IIC_RX_FIFO_DEPTH) {
 			/*
@@ -546,7 +548,7 @@ static void xiic_smbus_block_read_setup(struct xiic_i2c *i2c)
 			 * Receive fifo depth should set to Rx fifo capacity minus 1
 			 */
 			rfd_set = IIC_RX_FIFO_DEPTH - 1;
-			i2c->rx_msg->len = rxmsg_len + 1;
+			i2c->rx_msg->len = rxmsg_len + 1 + pec_len;
 		} else if ((rxmsg_len == 1) ||
 			(rxmsg_len == 0)) {
 			/*
@@ -562,7 +564,7 @@ static void xiic_smbus_block_read_setup(struct xiic_i2c *i2c)
 			 * Receive fifo depth should set to Rx msg len minus 2
 			 */
 			rfd_set = rxmsg_len - 2;
-			i2c->rx_msg->len = rxmsg_len + 1;
+			i2c->rx_msg->len = rxmsg_len + 1 + pec_len;
 		}
 		xiic_setreg8(i2c, XIIC_RFD_REG_OFFSET, rfd_set);
 

-- 
2.53.0