usb: typec: tcpm: Try PD-2.0 if sink does not respond to 3.0 source-caps

[Guenter Roeck <linux@roeck-us.net>]

On Fri, Mar 15, 2019 at 03:42:19PM +0100, Hans de Goede wrote:
> PD 2.0 sinks are supposed to accept src-capabilities with a 3.0 header and
> simply ignore any src PDOs which the sink does not understand such as PPS
> but some 2.0 sinks instead ignore the entire PD_DATA_SOURCE_CAP message,
> causing contract negotiation to fail.
> 
> This commit fixes such sinks not working by re-trying the contract
> negotiation with PD-2.0 source-caps messages if we don't have a contract
> after PD_N_HARD_RESET_COUNT hard-reset attempts.
> 
> The problem fixed by this commit was noticed with a Type-C to VGA dongle.
> 
> Signed-off-by: Hans de Goede <hdegoede@redhat.com>
> ---
> The Type-C to VGA dongle on which this encountered looks like this one:
> https://www.aliexpress.com/item/Male-USB-3-1-Type-C-USB-C-to-Female-VGA-Adapter-Cable-10Gbps-for-New/32898274476.html
> ---
>  drivers/usb/typec/tcpm/tcpm.c | 34 +++++++++++++++++++++++++++++++++-
>  1 file changed, 33 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/usb/typec/tcpm/tcpm.c b/drivers/usb/typec/tcpm/tcpm.c
> index f1c39a3c7534..3f8df845d1a5 100644
> --- a/drivers/usb/typec/tcpm/tcpm.c
> +++ b/drivers/usb/typec/tcpm/tcpm.c
> @@ -37,6 +37,7 @@
>  	S(SRC_ATTACHED),			\
>  	S(SRC_STARTUP),				\
>  	S(SRC_SEND_CAPABILITIES),		\
> +	S(SRC_SEND_CAP_LOWER_PD_REVISION),	\
>  	S(SRC_NEGOTIATE_CAPABILITIES),		\
>  	S(SRC_TRANSITION_SUPPLY),		\
>  	S(SRC_READY),				\
> @@ -2792,6 +2793,29 @@ static inline enum tcpm_state hard_reset_state(struct tcpm_port *port)
>  	return SNK_UNATTACHED;
>  }
>  
> +/*
> + * PD 2.0 sinks are supposed to accept src-capabilities with a 3.0 header and
> + * simply ignore any src PDOs which the sink does not understand such as PPS
> + * but some 2.0 sinks instead ignore the entire PD_DATA_SOURCE_CAP message,
> + * causing contract negotiation to fail.
> + *
> + * This function is used by the SRC_SEND_CAPABILITIES state in
> + * run_state_machine() to work around this.
> + *
> + * After PD_N_HARD_RESET_COUNT hard-reset attempts this function selects
> + * SRC_SEND_CAP_LOWER_PD_REVISION as state to set after the next timeout,
> + * this state will fallback to a lower PD revision and then try sending the
> + * src-capabilities again.
> + */
> +static inline enum tcpm_state src_send_cap_timeout_state(struct tcpm_port *port)
> +{
> +	if (port->hard_reset_count < PD_N_HARD_RESET_COUNT)
> +		return HARD_RESET_SEND;
> +	if (port->negotiated_rev > PD_REV20)
> +		return SRC_SEND_CAP_LOWER_PD_REVISION;
> +	return hard_reset_state(port);
> +}
> +
>  static inline enum tcpm_state unattached_state(struct tcpm_port *port)
>  {
>  	if (port->port_type == TYPEC_PORT_DRP) {
> @@ -2966,10 +2990,18 @@ static void run_state_machine(struct tcpm_port *port)
>  			/* port->hard_reset_count = 0; */
>  			port->caps_count = 0;
>  			port->pd_capable = true;
> -			tcpm_set_state_cond(port, hard_reset_state(port),
> +			tcpm_set_state_cond(port,
> +					    src_send_cap_timeout_state(port),
>  					    PD_T_SEND_SOURCE_CAP);
>  		}
>  		break;
> +	case SRC_SEND_CAP_LOWER_PD_REVISION:
> +		if (WARN_ON(port->negotiated_rev <= PD_REV20))
> +			break;

I really dislike the WARN_ON here. A bad remote can potentially trigger
this, which on systems with crash on warning enabled can result in a
reboot. Just revert to the original behavior here, and maybe add
a tcpm log message.

Guenter

> +		port->negotiated_rev--;
> +		port->hard_reset_count = 0;
> +		tcpm_set_state(port, SRC_SEND_CAPABILITIES, 0);
> +		break;
>  	case SRC_NEGOTIATE_CAPABILITIES:
>  		ret = tcpm_pd_check_request(port);
>  		if (ret < 0) {
> -- 
> 2.20.1
>