[PATCH 0/3] i2c: xiic: fix SMBus block read and PEC support

[Abdurrahman Hussain via B4 Relay]

This series fixes three independent bugs in the Xilinx AXI IIC driver
that together make SMBus block reads with PEC return -EBADMSG or -EIO
on otherwise clean transfers. They only surface when the client has
I2C_CLIENT_PEC set; non-PEC block reads happen to mask each issue in
turn.

The problems were uncovered driving an adm1266 PMBus device behind a
Xilinx AXI IIC FPGA block and reading its 64-byte blackbox record.

Patch 1 stops xiic_smbus_block_read_setup() from truncating rx_msg->len.
The i2c core appends a byte to msg->len when PEC is enabled, so
overwriting the length to "block size + 1" silently drops the PEC byte
and i2c_smbus_check_pec() then reads the last payload byte as the PEC.

Patch 2 raises the RX_FULL threshold so the interrupt only fires once
every remaining byte (payload plus optional PEC) is already buffered in
the FIFO. The previous threshold of rxmsg_len - 2 caused the
bytes_rem == 1 path in xiic_read_rx() to NACK a byte still on the wire.

Patch 3 stops the BNB handler from forcing tx_msg->len = 1 to signal
completion. tx_msg and rx_msg alias the same i2c_msg during a receive,
so this also clobbered rx_msg->len; and because tx_pos is already at 2
in the PEC case, the unsigned subtraction in xiic_tx_space() underflowed
and the STATE_DONE check fell through to STATE_ERROR. Advancing tx_pos
up to msg->len drives tx_space to zero without touching the length.

All three patches are pure bug fixes; non-PEC behaviour is unchanged.
Tested on real hardware -- a Xilinx AXI IIC controller talking to an
adm1266, where 64-byte PEC-checked block reads now complete cleanly.

Signed-off-by: Abdurrahman Hussain <abdurrahman at nexthop.ai>
---
Abdurrahman Hussain (3):
      i2c: xiic: preserve PEC byte length in SMBus block read setup
      i2c: xiic: defer RX_FULL until all trailing bytes are in FIFO
      i2c: xiic: don't clobber msg->len to signal block-read completion

 drivers/i2c/busses/i2c-xiic.c | 20 +++++++++++---------
 1 file changed, 11 insertions(+), 9 deletions(-)
---
base-commit: 254f49634ee16a731174d2ae34bc50bd5f45e731
change-id: 20260427-i2c-xiic-2aeb501ec02a

Best regards,
--  
Abdurrahman Hussain <abdurrahman at nexthop.ai>