Re: [PATCH v6 2/4] tty: rename tty_kopen() and add new function tty_kopen_shared() - Uwe Kleine-König

From: "Uwe Kleine-König" <u.kleine-koenig@pengutronix.de>
To: Johan Hovold <johan@kernel.org>
Cc: kernel@pengutronix.de, linux-serial@vger.kernel.org,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	linux-kernel@vger.kernel.org, Dan Murphy <dmurphy@ti.com>,
	Pavel Machek <pavel@ucw.cz>, Jiri Slaby <jslaby@suse.com>,
	linux-leds@vger.kernel.org,
	Jacek Anaszewski <jacek.anaszewski@gmail.com>
Subject: Re: [PATCH v6 2/4] tty: rename tty_kopen() and add new function tty_kopen_shared()
Date: Thu, 20 Feb 2020 12:04:27 +0100	[thread overview]
Message-ID: <20200220110427.e6jgzvdhh3ysql3n@pengutronix.de> (raw)
In-Reply-To: <20200219171759.GE32540@localhost>

On Wed, Feb 19, 2020 at 06:17:59PM +0100, Johan Hovold wrote:
> On Wed, Feb 19, 2020 at 05:37:58PM +0100, Uwe Kleine-König wrote:
> > On Wed, Feb 19, 2020 at 02:21:13PM +0100, Johan Hovold wrote:
> > > On Thu, Feb 13, 2020 at 10:15:58AM +0100, Uwe Kleine-König wrote:
> > > > From: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
> > > > 
> > > > Introduce a new function tty_kopen_shared() that yields a struct
> > > > tty_struct. The semantic difference to tty_kopen() is that the tty is
> > > > expected to be used already. So rename tty_kopen() to
> > > > tty_kopen_exclusive() for clearness, adapt the single user and put the
> > > > common code in a new static helper function.
> > > > 
> > > > tty_kopen_shared is to be used to implement an LED trigger for tty
> > > > devices in one of the next patches.
> > > > 
> > > > Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
> > > > ---
> > >  
> > > > -/**
> > > > - *	tty_kopen	-	open a tty device for kernel
> > > > - *	@device: dev_t of device to open
> > > > - *
> > > > - *	Opens tty exclusively for kernel. Performs the driver lookup,
> > > > - *	makes sure it's not already opened and performs the first-time
> > > > - *	tty initialization.
> > > > - *
> > > > - *	Returns the locked initialized &tty_struct
> > > > - *
> > > > - *	Claims the global tty_mutex to serialize:
> > > > - *	  - concurrent first-time tty initialization
> > > > - *	  - concurrent tty driver removal w/ lookup
> > > > - *	  - concurrent tty removal from driver table
> > > > - */
> > > > -struct tty_struct *tty_kopen(dev_t device)
> > > > +static struct tty_struct *tty_kopen(dev_t device, int shared)
> > > >  {
> > > >  	struct tty_struct *tty;
> > > >  	struct tty_driver *driver;
> > > > @@ -1905,7 +1890,7 @@ struct tty_struct *tty_kopen(dev_t device)
> > > >  
> > > >  	/* check whether we're reopening an existing tty */
> > > >  	tty = tty_driver_lookup_tty(driver, NULL, index);
> > > > -	if (IS_ERR(tty))
> > > > +	if (IS_ERR(tty) || shared)
> > > 
> > > So here you skip initialisation and return NULL if the tty isn't already
> > > in use (e.g. is open) when shared is set.
> > 
> > Which is good, right? If I remember my tests correctly this even works
> > if the tty isn't opened but just "exists".
> 
> No, this means that your trigger will never be installed unless the port
> is already open, yet the sysfs interface still returns success (see
> patch 4/4 dev_store()).

Ah I think I see. tty_driver_lookup_tty() might return NULL which for
the trigger driver indicates that tty_kopen_shared should be retried,
right?

> Note that the struct tty doesn't exist until the port is opened; it's
> allocated in tty_init_dev() that you skip above when "shared" is set.

That needs fixing. I will try to resolve the dialog with Andy before
addressing that in the next iteration.

Best regards
Uwe

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