[PATCH v3 0/3] i2c: xiic: fix SMBus block read and PEC support

[Abdurrahman Hussain]

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.
The chunk-vs-defer guard now also accounts for the PEC byte so a
rxmsg_len == IIC_RX_FIFO_DEPTH PEC-enabled read does not push
XIIC_RFD_REG_OFFSET past its 4-bit range.

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>
---
Changes in v3 (addresses the sashiko automated review of v2):
- Patch 1: handle short SMBus block reads where the controller pads
  rx_msg->len up to SMBUS_BLOCK_READ_MIN_LEN for its end-of-message
  workaround. In v2 this branch left the PEC byte at the padded
  offset rather than the actual end-of-payload, so the i2c core's
  PEC validator read past the chip data. Track the on-wire length
  in a new smbus_actual_len field populated in the minlen branch
  of xiic_smbus_block_read_setup(), and trim rx_msg->len back at
  RX_FULL completion before passing the message up. Addresses
  sashiko's v2 note about the pec_len adjustment missing the
  rxmsg_len < 3 padding branch; that branch was indeed the cause
  of pmbus_check_block_register() silently failing on zero-length
  MFR_* fields and skipping debugfs auto-discovery on affected
  hardware.
- Patch 3: defensively reset smbus_actual_len in the BNB completion
  handler so a subsequent non-SMBus transfer cannot see a stale
  trim value from a completed short block read.
- Patch 2 is unchanged from v2. sashiko's two other v2 notes were
  investigated and judged not to require code changes: the concern
  about removed padding in the chunked-vs-deferred drain misread
  the patch (the padding survives via the else branch and the new
  PEC-aware guard preserves the original semantics), and the
  flagged unsigned underflow in xiic_tx_space() is unreachable
  because tx_pos is bounded by tx_msg->len at the call site.
- Link to v2: https://patch.msgid.link/20260511-i2c-xiic-v2-0-c16380cb1594@nexthop.ai

Changes in v2:
- Patch 2: widen the chunk-vs-defer guard in xiic_smbus_block_read_setup()
  to include pec_len, so a 16-byte PEC-enabled block read routes through
  the chunked drain rather than writing 16 into the 4-bit
  XIIC_RFD_REG_OFFSET register. No tree-level change to patches 1 or 3.
- Link to v1: https://patch.msgid.link/20260427-i2c-xiic-v1-0-e6207f9aa5ad@nexthop.ai

To: Michal Simek <michal.simek at amd.com>
To: Andi Shyti <andi.shyti at kernel.org>
Cc: linux-arm-kernel at lists.infradead.org
Cc: linux-i2c at vger.kernel.org
Cc: linux-kernel at vger.kernel.org

---
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 | 67 ++++++++++++++++++++++++++++++++-----------
 1 file changed, 50 insertions(+), 17 deletions(-)
---
base-commit: 254f49634ee16a731174d2ae34bc50bd5f45e731
change-id: 20260427-i2c-xiic-2aeb501ec02a

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