usb: typec: tcpm: Allow slow charging loops to comply to pSnkStby

When a PD charger advertising Rp-3.0 is connected to a sink port, the
sink port current limit would 3A, during SNK_DISCOVERY, till power
negotiation starts. Once the negotiation starts the power limit needs
to drop down to pSnkStby(500mA @ 5V) and to negotiated current limit
once the explicit contract is in place. Not all charging loops can ramp
up to 3A and drop down to 500mA within tSnkStdby which is 15ms. The port
partner might hard reset if tSnkStdby is not met.

To solve this problem, this patch introduces slow-charger-loop which
when set makes the port request PD_P_SNK_STDBY_MW upon entering
SNK_DISCOVERY(instead of 3A or the 1.5A during SNK_DISCOVERY) and the
actual currrent limit after RX of PD_CTRL_PSRDY for PD link or during
SNK_READY for non-pd link.

Reviewed-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Signed-off-by: Badhri Jagan Sridharan <badhri@google.com>
Link: https://lore.kernel.org/r/20210414024000.4175263-2-badhri@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/typec/tcpm/tcpm.c

@@ -459,6 +459,12 @@ struct tcpm_port {
 	/* Auto vbus discharge status */
 	bool auto_vbus_discharge_enabled;
 
+	/*
+	 * When set, port requests PD_P_SNK_STDBY_MW upon entering SNK_DISCOVERY and
+	 * the actual currrent limit after RX of PD_CTRL_PSRDY for PD link,
+	 * SNK_READY for non-pd link.
+	 */
+	bool slow_charger_loop;
 #ifdef CONFIG_DEBUG_FS
 	struct dentry *dentry;
 	struct mutex logbuffer_lock;	/* log buffer access lock */
@@ -4047,9 +4053,11 @@ static void run_state_machine(struct tcpm_port *port)
 		break;
 	case SNK_DISCOVERY:
 		if (port->vbus_present) {
-			tcpm_set_current_limit(port,
-					       tcpm_get_current_limit(port),
-					       5000);
+			u32 current_lim = tcpm_get_current_limit(port);
+
+			if (port->slow_charger_loop || (current_lim > PD_P_SNK_STDBY_MW / 5))
+				current_lim = PD_P_SNK_STDBY_MW / 5;
+			tcpm_set_current_limit(port, current_lim, 5000);
 			tcpm_set_charge(port, true);
 			tcpm_set_state(port, SNK_WAIT_CAPABILITIES, 0);
 			break;
@@ -4161,6 +4169,8 @@ static void run_state_machine(struct tcpm_port *port)
 			port->pwr_opmode = TYPEC_PWR_MODE_PD;
 		}
 
+		if (!port->pd_capable && port->slow_charger_loop)
+			tcpm_set_current_limit(port, tcpm_get_current_limit(port), 5000);
 		tcpm_swap_complete(port, 0);
 		tcpm_typec_connect(port);
 		mod_enable_frs_delayed_work(port, 0);
@@ -5763,6 +5773,7 @@ static int tcpm_fw_get_caps(struct tcpm_port *port,
 	port->typec_caps.type = ret;
 	port->port_type = port->typec_caps.type;
 
+	port->slow_charger_loop = fwnode_property_read_bool(fwnode, "slow-charger-loop");
 	if (port->port_type == TYPEC_PORT_SNK)
 		goto sink;