linux-kernel - Re: [PATCH v6 2/4] tty: rename tty_kopen() and add new function tty_kopen

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Date:   Wed, 19 Feb 2020 17:37:58 +0100
From:   Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>
To:     Johan Hovold <johan@...nel.org>
Cc:     Jacek Anaszewski <jacek.anaszewski@...il.com>,
        Pavel Machek <pavel@....cz>, Dan Murphy <dmurphy@...com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.com>, linux-leds@...r.kernel.org,
        linux-kernel@...r.kernel.org, kernel@...gutronix.de,
        linux-serial@...r.kernel.org
Subject: Re: [PATCH v6 2/4] tty: rename tty_kopen() and add new function
 tty_kopen_shared()

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@...gutronix.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@...gutronix.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".

> >  		goto out;
> >  
> >  	if (tty) {
> > @@ -1923,7 +1908,44 @@ struct tty_struct *tty_kopen(dev_t device)
> >  	tty_driver_kref_put(driver);
> >  	return tty;
> >  }
> > -EXPORT_SYMBOL_GPL(tty_kopen);
> > +
> > +/**
> > + *	tty_kopen_exclusive	-	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_exclusive(dev_t device)
> > +{
> > +	return tty_kopen(device, 0);
> > +}
> > +EXPORT_SYMBOL_GPL(tty_kopen_exclusive);
> > +
> > +/**
> > + *	tty_kopen_shared	-	open a tty device for shared in-kernel use
> > + *	@device: dev_t of device to open
> > + *
> > + *	Opens an already existing tty
> > + *	rnel. Performs the driver lookup,
> 
> "rnel"?
> 
> > + *	makes sure it's not already opened and performs the first-time
> > + *	tty initialization.
> 
> Yet, you claim to do initialisation here, which isn't the case.

Yeah, wrong (and incomplete) copy of the tty_kopen_exclusive docstring.

> > + *
> > + *	Locking is identical to tty_kopen() above.
> > + */
> > +struct tty_struct *tty_kopen_shared(dev_t device)
> > +{
> > +	return tty_kopen(device, 1);
> > +}
> > +EXPORT_SYMBOL_GPL(tty_kopen_shared);
> 
> This "kopen" naming is unfortunate as the tty isn't really opened by
> either of these functions, but that's not something you introduced.

Ack, will send a v7 with the doc string fixed.

Thanks for taking the time to look over 
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | https://www.pengutronix.de/ |