[PATCH v2 2/3] i2c: xiic: defer RX_FULL until all trailing bytes are in FIFO

[Abdurrahman Hussain]

For the normal path of xiic_smbus_block_read_setup() (rxmsg_len less
than IIC_RX_FIFO_DEPTH), RFD was programmed to rxmsg_len - 2, which
fires the RX_FULL interrupt while the last payload byte is still in
flight. xiic_read_rx()'s bytes_rem == 1 branch then sets NACK on that
byte still on the wire, truncating the read in the PEC-enabled case.

Raise the threshold so RX_FULL fires only once every remaining byte
(payload plus optional PEC) is already buffered in the FIFO. That
routes the drain through xiic_read_rx()'s bytes_rem == 0 path, which
reads everything out and emits the stop cleanly. For the non-PEC path
the full payload is still read out through the same bytes_rem == 0
branch; the only user-visible change is that the controller waits one
extra byte-time before servicing the interrupt.

The deferred-fire formula is rxmsg_len + pec_len - 1, and the RFD
register at XIIC_RFD_REG_OFFSET is a 4-bit field. Widen the
chunk-vs-defer guard to (rxmsg_len + pec_len > IIC_RX_FIFO_DEPTH) so
the boundary case rxmsg_len == IIC_RX_FIFO_DEPTH with PEC enabled
cannot write 16 into that 4-bit register; it routes through the
chunked drain instead, which already caps RFD at IIC_RX_FIFO_DEPTH - 1.

Signed-off-by: Abdurrahman Hussain <abdurrahman at nexthop.ai>
---
 drivers/i2c/busses/i2c-xiic.c | 14 ++++++--------
 1 file changed, 6 insertions(+), 8 deletions(-)

diff --git a/drivers/i2c/busses/i2c-xiic.c b/drivers/i2c/busses/i2c-xiic.c
index 959a47b645a5..a9452eca26df 100644
--- a/drivers/i2c/busses/i2c-xiic.c
+++ b/drivers/i2c/busses/i2c-xiic.c
@@ -542,10 +542,11 @@ static void xiic_smbus_block_read_setup(struct xiic_i2c *i2c)
 		unsigned int pec_len = i2c->rx_msg->len - 1;
 
 		/* Set Receive fifo depth */
-		if (rxmsg_len > IIC_RX_FIFO_DEPTH) {
+		if (rxmsg_len + pec_len > IIC_RX_FIFO_DEPTH) {
 			/*
-			 * When Rx msg len greater than or equal to Rx fifo capacity
-			 * Receive fifo depth should set to Rx fifo capacity minus 1
+			 * Trailing payload (data + optional PEC) exceeds Rx FIFO
+			 * capacity; drain in chunks. Fire RX_FULL when the FIFO is
+			 * full and let the ISR re-arm for the remainder.
 			 */
 			rfd_set = IIC_RX_FIFO_DEPTH - 1;
 			i2c->rx_msg->len = rxmsg_len + 1 + pec_len;
@@ -559,11 +560,8 @@ static void xiic_smbus_block_read_setup(struct xiic_i2c *i2c)
 			rfd_set = 0;
 			i2c->rx_msg->len = SMBUS_BLOCK_READ_MIN_LEN;
 		} else {
-			/*
-			 * When Rx msg len less than Rx fifo capacity
-			 * Receive fifo depth should set to Rx msg len minus 2
-			 */
-			rfd_set = rxmsg_len - 2;
+			/* Defer RX_FULL until all trailing bytes are in FIFO. */
+			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);

-- 
2.53.0