[PATCH] usb: typec: fusb302: Send PD messages longer than one SMBus block

[Alexey Charkov]

The whole message, tokens included, goes to the FIFO in a single SMBus
block write, which carries at most 32 bytes and silently drops the rest.
A message with six or seven data objects needs up to 39, so it loses its
closing tokens including TXON: the chip never sends it, the transmission
times out, and the truncated bytes stay in the TX FIFO ahead of the next
message. A Discover Identity ACK carrying a full set of sink-vdos is one
such message, as is a Source_Capabilities with six or seven PDOs.

Write the message in chunks of at most one SMBus block. The FIFOS
register has no address auto-increment and queues every byte written to
it, so consecutive writes append to the 48-byte TX FIFO. TXON is the
one token that is not queued but starts the transmitter as soon as it is
written. It still comes last, so transmission begins only once the whole
message is in the FIFO, as the datasheet recommends.

Flush the TX FIFO if a write fails, so that a partial message cannot
stay queued and go out together with the next one as a garbled frame.

Signed-off-by: Alexey Charkov <alchark@xxxxxxxxxxx>
---
 drivers/usb/typec/tcpm/fusb302.c | 21 ++++++++++++++++++---
 1 file changed, 18 insertions(+), 3 deletions(-)

diff --git a/drivers/usb/typec/tcpm/fusb302.c b/drivers/usb/typec/tcpm/fusb302.c
index 3319f6a2b0c9..e10b7a4b679f 100644
--- a/drivers/usb/typec/tcpm/fusb302.c
+++ b/drivers/usb/typec/tcpm/fusb302.c
@@ -967,6 +967,7 @@ static int fusb302_pd_send_message(struct fusb302_chip *chip,
 	int ret = 0;
 	u8 buf[40];
 	u8 pos = 0;
+	u8 i;
 	int len;
 
 	/* SOP tokens */
@@ -1001,9 +1002,23 @@ static int fusb302_pd_send_message(struct fusb302_chip *chip,
 	/* start transmission */
 	buf[pos++] = FUSB302_TKN_TXON;
 
-	ret = fusb302_i2c_block_write(chip, FUSB_REG_FIFOS, pos, buf);
-	if (ret < 0)
-		return ret;
+	/*
+	 * An SMBus block write carries at most I2C_SMBUS_BLOCK_MAX bytes, and
+	 * a message with more than five data objects needs more. The FIFO
+	 * register queues every byte written to it except TXON, which starts
+	 * the transmitter instead. TXON comes last, so the message can go out
+	 * in several writes.
+	 */
+	for (i = 0; i < pos; i += I2C_SMBUS_BLOCK_MAX) {
+		ret = fusb302_i2c_block_write(chip, FUSB_REG_FIFOS,
+					      min_t(u8, pos - i, I2C_SMBUS_BLOCK_MAX),
+					      &buf[i]);
+		if (ret < 0) {
+			/* don't leave a partial message ahead of the next one */
+			fusb302_pd_tx_flush(chip);
+			return ret;
+		}
+	}
 	fusb302_log(chip, "sending PD message header: %x", msg->header);
 	fusb302_log(chip, "sending PD message len: %d", len);
 

---
base-commit: 7649ba73f9e7e1e04d0b46213e39c79ae09dd55c
change-id: 20261102-fusb302-long-tx-d2d06ed2a937

Best regards,
--  
Alexey Charkov <alchark@xxxxxxxxxxx>