Re: [PATCH 2/2] typec: tcpm: Add option to maintain current limit at Vsafe5V

[Jack Pham <jackp@codeaurora.org>]

Hello Badhri,

On Wed, Sep 12, 2018 at 07:11:13PM -0700, Badhri Jagan Sridharan wrote:
> During hard reset, TCPM turns off the charging path.
> The spec provides an option for Sink to either drop to vSafe5V or vSafe0V.

This doesn't make sense. By definition the sink isn't sourcing VBUS, so
how can it control whether to allow the voltage to be 5V or 0V?

> From USB_PD_R3_0
> 2.6.2 Sink Operation
> ..
> Serious errors are handled by Hard Reset Signaling issued by either Port
> Partner. A Hard Reset:
> resets protocol as for a Soft Reset but also returns the power supply to
> USB Default Operation (vSafe0V or vSafe5V output) in order to protect the
> Sink.

I can see how the wording here "vSafe0V *or* vSafe5V" is misleading, as
I think it actually is both. In later parts of the spec, the source's
VBUS behavior is well defined in that it must first drop to vSafe0V
and then return to vSafe5V. Please refer to section 7.1.5.

> Add a config option to tcpc_dev and let the device specific driver decide
> what needs to be done.
> 
> Signed-off-by: Badhri Jagan Sridharan <Badhri@google.com>
> ---
>  drivers/usb/typec/tcpm.c | 7 ++++++-
>  include/linux/usb/tcpm.h | 1 +
>  2 files changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/usb/typec/tcpm.c b/drivers/usb/typec/tcpm.c
> index a4e0c027a2a9..350d1a7c4543 100644
> --- a/drivers/usb/typec/tcpm.c
> +++ b/drivers/usb/typec/tcpm.c
> @@ -3269,7 +3269,12 @@ static void run_state_machine(struct tcpm_port *port)
>  	case SNK_HARD_RESET_SINK_OFF:
>  		memset(&port->pps_data, 0, sizeof(port->pps_data));
>  		tcpm_set_vconn(port, false);
> -		tcpm_set_charge(port, false);
> +		if (port->tcpc->config->vsafe_5v_hard_reset)

Therefore I think this config option doesn't make sense.

> +			tcpm_set_current_limit(port,
> +					       tcpm_get_current_limit(port),
> +					       5000);

But I do think this might still be useful at least in ensuring the sink
returns to drawing only default power during the transition before
re-establishing a contract. Given that the sink can't control when
exactly VBUS will go to 0V, is it ok to call set_current_limit() even if
VBUS is momentarily 0V, so at least it is in preparation for when VBUS
turns back on? Or would it be safer to do it during the
SNK_HARD_RESET_SINK_ON state after we know VBUS is back to vSafe5V?

> +		else
> +			tcpm_set_charge(port, false);

Regards,
Jack
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