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

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Message-ID: <20200310150834.GA24886@duo.ucw.cz>
Date:   Tue, 10 Mar 2020 16:08:35 +0100
From:   Pavel Machek <pavel@...x.de>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Sergey Organov <sorganov@...il.com>,
        Michał Mirosław <mirq-linux@...e.qmqm.pl>,
        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

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?
								Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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