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

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Date:   Mon, 17 May 2021 13:03:09 +0300
From:   Heikki Krogerus <heikki.krogerus@...ux.intel.com>
To:     Bjorn Andersson <bjorn.andersson@...aro.org>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Benjamin Berg <bberg@...hat.com>, linux-usb@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] usb: typec: ucsi: Clear pending after acking connector
 change

On Sat, May 15, 2021 at 09:09:53PM -0700, Bjorn Andersson wrote:
> 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 until 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 <bjorn.andersson@...aro.org>

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.

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.

> ---
>  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,

-- 
heikki