Re: [PATCH] usb: typec: ucsi: Clear pending after acking connector change

On Mon, 2021-05-17 at 13:03 +0300, Heikki Krogerus wrote:

On Sat, May 15, 2021 at 09:09:53PM -0700, Bjorn Andersson wrote:

quoted

It's possible that the interrupt handler for the UCSI driver
signals a
connector changes after the handler clears the PENDING bit, but
before
it has sent the acknowledge request. The result is that the handler
is
invoked yet again, to ack the same connector change.

At least some versions of the Qualcomm UCSI firmware will not
handle the
second - "spurious" - acknowledgment gracefully. So make sure to
not
clear the pending flag untuntil the changeil the change is
acknowledged.

Any connector changes coming in after the acknowledgment, that
would
have the pending flag incorrectly cleared, would afaict be covered
by
the subsequent connector status check.

Fixes: 217504a05532 ("usb: typec: ucsi: Work around PPM losing
change information")
Signed-off-by: Bjorn Andersson <redacted>

I'm OK with this if Bejamin does not see any problems with it. I'll
wait for his comments before giving my reviewed-by tag.

Interesting problem. I wonder if we can detect this situation and
restart the worker instead of sending the ACK.

As for the patch, I do believe it is safe. I agree with the assessment
in the commit message, that the subsequent connector check will detect
any race condition and recovers from it.

Acked-By: Benjamin Berg <redacted>

Benjamin

That workaround (commit 217504a05532) is unfortunately too fragile.
I'm going to now separate the processing of the connector state from
the event handler (interrupt handler). That way we should be fairly
sure we don't loose any of the connector states even if an event is
generated while we are still in the middle of processing the previous
one(s), and at the same time be sure that we also don't confuse the
firmware.
So the event handler shall after that only read the connector status,
schedule the unique job where it's processed and ACK the event.
Nothing else.

quoted

---
 drivers/usb/typec/ucsi/ucsi.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/usb/typec/ucsi/ucsi.c

b/drivers/usb/typec/ucsi/ucsi.c
index 282c3c825c13..f451ce0132a9 100644

--- a/drivers/usb/typec/ucsi/ucsi.c
+++ b/drivers/usb/typec/ucsi/ucsi.c

@@ -694,8 +694,8 @@ static void ucsi_handle_connector_change(struct

work_struct *work)
        ucsi_send_command(con->ucsi, command, NULL, 0);
 
        /* 3. ACK connector change */
-       clear_bit(EVENT_PENDING, &ucsi->flags);
        ret = ucsi_acknowledge_connector_change(ucsi);
+       clear_bit(EVENT_PENDING, &ucsi->flags);
        if (ret) {
                dev_err(ucsi->dev, "%s: ACK failed (%d)", __func__,
ret);
                goto out_unlock;
-- 
2.29.2

thanks,