linux-kernel - Re: [PATCH 4.19 12/86] usb: gadget: serial: fix Tx stall after buffer overflow

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Message-ID: <20200310225118.GA32479@qmqm.qmqm.pl>
Date:   Tue, 10 Mar 2020 23:51:18 +0100
From:   Michał Mirosław <mirq-linux@...e.qmqm.pl>
To:     Pavel Machek <pavel@...x.de>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Sergey Organov <sorganov@...il.com>,
        Felipe Balbi <balbi@...nel.org>,
        Sasha Levin <sashal@...nel.org>
Subject: Re: [PATCH 4.19 12/86] usb: gadget: serial: fix Tx stall after
 buffer overflow

On Tue, Mar 10, 2020 at 04:08:35PM +0100, Pavel Machek wrote:
> Hi!
> 
> > From: Sergey Organov <sorganov@...il.com>
> > 
> > [ Upstream commit e4bfded56cf39b8d02733c1e6ef546b97961e18a ]
> > 
> > Symptom: application opens /dev/ttyGS0 and starts sending (writing) to
> > it while either USB cable is not connected, or nobody listens on the
> > other side of the cable. If driver circular buffer overflows before
> > connection is established, no data will be written to the USB layer
> > until/unless /dev/ttyGS0 is closed and re-opened again by the
> > application (the latter besides having no means of being notified about
> > the event of establishing of the connection.)
> > 
> > Fix: on open and/or connect, kick Tx to flush circular buffer data to
> > USB layer.
> 
> > diff --git a/drivers/usb/gadget/function/u_serial.c b/drivers/usb/gadget/function/u_serial.c
> > index d4d317db89df5..38afe96c5cd26 100644
> > --- a/drivers/usb/gadget/function/u_serial.c
> > +++ b/drivers/usb/gadget/function/u_serial.c
> > @@ -567,8 +567,10 @@ static int gs_start_io(struct gs_port *port)
> >  	port->n_read = 0;
> >  	started = gs_start_rx(port);
> >  
> > -	/* unblock any pending writes into our circular buffer */
> >  	if (started) {
> > +		gs_start_tx(port);
> > +		/* Unblock any pending writes into our circular buffer, in case
> > +		 * we didn't in gs_start_tx() */
> >  		tty_wakeup(port->port.tty);
> 
> I'm confused. gs-start_tx() is done twice in a row. Its return
> convention seem to be 0 in success case, and non-zero on failure. But
> it is assigned to variable called "started", which does not sound like
> "error" to me.
> 
> Are you sure this is correct?

The function before 'if (started)' is gs_start_rx() - it's RX not TX.

Best Regards,
Michał Mirosław