usb: typec: fusb302: cache PD RX state

This patch fixes a race condition communication error, which ends up in
PD hard resets when losing the race. Some systems, like the Radxa ROCK
5B are powered through USB-C without any backup power source and use a
FUSB302 chip to do the PD negotiation. This means it is quite important
to avoid hard resets, since that effectively kills the system's
power-supply.

I've found the following race condition while debugging unplanned power
loss during booting the board every now and then:

1. lots of TCPM/FUSB302/PD initialization stuff
2. TCPM ends up in SNK_WAIT_CAPABILITIES (tcpm_set_pd_rx is enabled here)
3. the remote PD source does not send anything, so TCPM does a SOFT RESET
4. TCPM ends up in SNK_WAIT_CAPABILITIES for the second time
   (tcpm_set_pd_rx is enabled again, even though it is still on)

At this point I've seen broken CRC good messages being send by the
FUSB302 with a logic analyzer sniffing the CC lines. Also it looks like
messages are being lost and things generally going haywire with one of
the two sides doing a hard reset once a broken CRC good message was send
to the bus.

I think the system is running into a race condition, that the FIFOs are
being cleared and/or the automatic good CRC message generation flag is
being updated while a message is already arriving.

Let's avoid this by caching the PD RX enabled state, as we have already
processed anything in the FIFOs and are in a good state. As a side
effect that this also optimizes I2C bus usage :)

As far as I can tell the problem theoretically also exists when TCPM
enters SNK_WAIT_CAPABILITIES the first time, but I believe this is less
critical for the following reason:

On devices like the ROCK 5B, which are powered through a TCPM backed
USB-C port, the bootloader must have done some prior PD communication
(initial communication must happen within 5 seconds after plugging the
USB-C plug). This means the first time the kernel TCPM state machine
reaches SNK_WAIT_CAPABILITIES, the remote side is not sending messages
actively. On other devices a hard reset simply adds some extra delay and
things should be good afterwards.

Fixes: c034a43 ("staging: typec: Fairchild FUSB302 Type-c chip driver")
Cc: stable <[email protected]>
Signed-off-by: Sebastian Reichel <[email protected]>
Reviewed-by: Heikki Krogerus <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Greg Kroah-Hartman <[email protected]>

Author: sre

drivers/usb/typec/tcpm/fusb302.c

@@ -104,6 +104,7 @@ struct fusb302_chip {
 	bool vconn_on;
 	bool vbus_on;
 	bool charge_on;
+	bool pd_rx_on;
 	bool vbus_present;
 	enum typec_cc_polarity cc_polarity;
 	enum typec_cc_status cc1;
@@ -841,6 +842,11 @@ static int tcpm_set_pd_rx(struct tcpc_dev *dev, bool on)
 	int ret = 0;
 
 	mutex_lock(&chip->lock);
+	if (chip->pd_rx_on == on) {
+		fusb302_log(chip, "pd is already %s", str_on_off(on));
+		goto done;
+	}
+
 	ret = fusb302_pd_rx_flush(chip);
 	if (ret < 0) {
 		fusb302_log(chip, "cannot flush pd rx buffer, ret=%d", ret);
@@ -863,6 +869,8 @@ static int tcpm_set_pd_rx(struct tcpc_dev *dev, bool on)
 			    str_on_off(on), ret);
 		goto done;
 	}
+
+	chip->pd_rx_on = on;
 	fusb302_log(chip, "pd := %s", str_on_off(on));
 done:
 	mutex_unlock(&chip->lock);