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

[Heikki Krogerus <heikki.krogerus@linux.intel.com>]

On Mon, Nov 02, 2026 at 05:44:38PM +0400, Alexey Charkov wrote:
> 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@flipper.net>

Reviewed-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>

> ---
>  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@flipper.net>
> 

-- 
heikki