Re: [PATCH] usb: typec: fusb302: Drop unacknowledged Get_Source_Cap_Extended

[Heikki Krogerus]

On Mon, Nov 02, 2026 at 05:09:05PM +0400, Alexey Charkov wrote:
> The chip mistakes Get_Source_Cap_Extended for a GoodCRC, as it apparently
> decodes only the low four bits of the message type. As a result, it sends
> no GoodCRC for it, but still queues it in the RX FIFO. The partner never
> sees the message as delivered due to no GoodCRC, so it retries and then
> sends a Soft_Reset. When FUSB302 receives the Soft_Reset, it finally
> raises its interrupt, at which point TCPM sees Get_Source_Cap_Extended in
> the RX FIFO instead of the Soft_Reset it just received, and answers it
> Not_Supported, which is not a valid reply to Soft_Reset, so the partner
> escalates to a hard reset instead of getting a chance to gracefully handle
> a lost Get_Source_Cap_Extended.
> 
> Discard Get_Source_Cap_Extended, which FUSB302 can never properly receive
> due to its inability to automatically GoodCRC it, and answering it from
> the host within the protocol specified 250us window without so much as an
> interrupt cannot be made to work.
> 
> This way the TCPM view of the PD exchange at least matches what the
> partner saw on the wire. This lets the TCPM complete a PD negotiation over
> the FUSB302 with e.g. an Apple Studio Display, which otherwise always ends
> in hard resets.
> 
> Signed-off-by: Alexey Charkov <alchark@xxxxxxxxxxx>

Acked-by: Heikki Krogerus <heikki.krogerus@xxxxxxxxxxxxxxx>

> ---
>  drivers/usb/typec/tcpm/fusb302.c | 13 +++++++++++++
>  1 file changed, 13 insertions(+)
> 
> diff --git a/drivers/usb/typec/tcpm/fusb302.c b/drivers/usb/typec/tcpm/fusb302.c
> index 3319f6a2b0c9..96dc48250d6a 100644
> --- a/drivers/usb/typec/tcpm/fusb302.c
> +++ b/drivers/usb/typec/tcpm/fusb302.c
> @@ -1463,6 +1463,19 @@ static int fusb302_pd_read_message(struct fusb302_chip *chip,
>  	fusb302_log(chip, "PD message header: %x", msg->header);
>  	fusb302_log(chip, "PD message len: %d", len);
>  
> +	/*
> +	 * The chip only decodes the low four bits of the message type, so it
> +	 * mistakes Get_Source_Cap_Extended (17) for a GoodCRC (1): it neither
> +	 * acknowledges it nor raises GCRCSENT, yet still queues it in the RX
> +	 * FIFO. Without a GoodCRC the partner never considers it delivered and
> +	 * will retry and then soft reset, so it must not be answered.
> +	 */
> +	if ((!len) &&
> +	    (pd_header_type_le(msg->header) == PD_CTRL_GET_SOURCE_CAP_EXT)) {
> +		fusb302_log(chip, "dropping unacknowledged PD message");
> +		return ret;
> +	}
> +
>  	/*
>  	 * Check if we've read off a GoodCRC message. If so then indicate to
>  	 * TCPM that the previous transmission has completed. Otherwise we pass
> 
> ---
> base-commit: 7649ba73f9e7e1e04d0b46213e39c79ae09dd55c
> change-id: 20261102-fusb302-getsourcecap-01c38dccb230
> 
> Best regards,
> --  
> Alexey Charkov <alchark@xxxxxxxxxxx>

-- 
heikki