Date:   Tue, 12 Mar 2019 21:52:13 +0000
From:   Calvin Owens <calvinowens@...com>
To:     Petr Mladek <pmladek@...e.com>
CC:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jonathan Corbet <corbet@....net>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-serial@...r.kernel.org" <linux-serial@...r.kernel.org>
Subject: Re: [PATCH 3/4] printk: Add consoles to a virtual "console" bus

On Monday 03/11 at 14:33 +0100, Petr Mladek wrote:
> On Fri 2019-03-01 16:48:19, Calvin Owens wrote:
> > This patch embeds a device struct in the console struct, and registers
> > them on a "console" bus so we can expose attributes in sysfs.
> > 
> > Currently, most drivers declare static console structs, and that is
> > incompatible with the dev refcount model. So we end up needing to patch
> > all of the console drivers to:
> > 
> > 	1. Dynamically allocate the console struct using a new helper
> > 	2. Handle the allocation in (1) possibly failing
> > 	3. Dispose of (1) with put_device()
> > 
> > Early console structures must still be static, since they're required
> > before we're able to allocate memory. The least ugly way I can come up
> > with to handle this is an "is_static" flag in the structure which makes
> > the gets and puts NOPs, and is checked in ->release() to catch mistakes.
> > 
> > diff --git a/drivers/char/lp.c b/drivers/char/lp.c
> > index 5c8d780637bd..e09cb192a469 100644
> > --- a/drivers/char/lp.c
> > +++ b/drivers/char/lp.c
> > @@ -857,12 +857,12 @@ static void lp_console_write(struct console *co, const char *s,
> >  	parport_release(dev);
> >  }
> >  
> > -static struct console lpcons = {
> > -	.name		= "lp",
> > +static const struct console_operations lp_cons_ops = {
> >  	.write		= lp_console_write,
> > -	.flags		= CON_PRINTBUFFER,
> >  };
> >  
> > +static struct console *lpcons;
> 
> I have got the following compilation error (see below):
> 
>   CC      drivers/char/lp.o
> drivers/char/lp.c: In function ‘lp_register’:
> drivers/char/lp.c:925:2: error: ‘lpcons’ undeclared (first use in this function)
>   lpcons = allocate_console_dfl(&lp_cons_ops, "lp", NULL);
>   ^
> drivers/char/lp.c:925:2: note: each undeclared identifier is reported only once for each function it appears in
> In file included from drivers/char/lp.c:125:0:
> drivers/char/lp.c:925:33: error: ‘lp_cons_ops’ undeclared (first use in this function)

D'oh, will fix.
 
> 
> >  #endif /* console on line printer */
> >  
> >  /* --- initialisation code ------------------------------------- */
> > @@ -921,6 +921,11 @@ static int lp_register(int nr, struct parport *port)
> >  						      &ppdev_cb, nr);
> >  	if (lp_table[nr].dev == NULL)
> >  		return 1;
> > +
> > +	lpcons = allocate_console_dfl(&lp_cons_ops, "lp", NULL);
> > +	if (!lpcons)
> > +		return -ENOMEM;
> 
> This should be done inside #ifdef CONFIG_LP_CONSOLE
> to avoid the above compilation error.
> 
> > +
> >  	lp_table[nr].flags |= LP_EXIST;
> >  
> >  	if (reset)
> 
> [...]
> > diff --git a/include/linux/console.h b/include/linux/console.h
> > index 3c27a4a29b8c..382591683033 100644
> > --- a/include/linux/console.h
> > +++ b/include/linux/console.h
> > @@ -142,20 +143,28 @@ static inline int con_debug_leave(void)
> >  #define CON_BRL		(32) /* Used for a braille device */
> >  #define CON_EXTENDED	(64) /* Use the extended output format a la /dev/kmsg */
> >  
> > -struct console {
> > -	char	name[16];
> > +struct console;
> > +
> > +struct console_operations {
> >  	void	(*write)(struct console *, const char *, unsigned);
> >  	int	(*read)(struct console *, char *, unsigned);
> >  	struct tty_driver *(*device)(struct console *, int *);
> >  	void	(*unblank)(void);
> >  	int	(*setup)(struct console *, char *);
> >  	int	(*match)(struct console *, char *name, int idx, char *options);
> > +};
> > +
> > +struct console {
> > +	char	name[16];
> >  	short	flags;
> >  	short	index;
> >  	int	cflag;
> >  	void	*data;
> >  	struct	 console *next;
> >  	int	level;
> > +	const struct console_operations *ops;
> > +	struct device dev;
> > +	int is_static;
> >  };
> >  
> >  /*
> > @@ -167,6 +176,29 @@ struct console {
> >  extern int console_set_on_cmdline;
> >  extern struct console *early_console;
> >  
> > +extern struct console *allocate_console(const struct console_operations *ops,
> > +					const char *name, short flags,
> > +					short index, void *data);
> > +
> > +#define allocate_console_dfl(ops, name, data) \
> > +	allocate_console(ops, name, CON_PRINTBUFFER, -1, data)
> > +
> > +/*
> > + * Helpers for get/put that do the right thing for static early consoles.
> > + */
> > +
> > +#define get_console(con) \
> > +do { \
> > +	if (!con->is_static) \
> > +		get_device(&(con)->dev); \
> > +} while (0)
> > +
> > +#define put_console(con) \
> > +do { \
> > +	if (con && !con->is_static) \
> > +		put_device(&((struct console *)con)->dev); \
> > +} while (0)
> > +
> >  extern int add_preferred_console(char *name, int idx, char *options);
> >  extern void register_console(struct console *);
> >  extern int unregister_console(struct console *);
> > diff --git a/include/linux/serial_core.h b/include/linux/serial_core.h
> > index 5fe2b037e833..29b43c4df3d6 100644
> > --- a/include/linux/serial_core.h
> > +++ b/include/linux/serial_core.h
> > @@ -426,6 +426,25 @@ int uart_add_one_port(struct uart_driver *reg, struct uart_port *port);
> >  int uart_remove_one_port(struct uart_driver *reg, struct uart_port *port);
> >  int uart_match_port(struct uart_port *port1, struct uart_port *port2);
> >  
> > +/*
> > + * This ugliness removes the need for #ifdef boilerplate in UART drivers which
> > + * allow their console functionality to be disabled via Kconfig.
> > + */
> > +#define uart_allocate_console(drv, ops, name, flags, idx, kcfg)		    \
> > +({									    \
> > +	int __retval = 0;						    \
> > +	if (IS_ENABLED(CONFIG_##kcfg)) {				    \
> 
> I wonder if it is worth it. I do not see much existing #ifdef's
> removed by this patch. I would expect that the code is never
> called when the driver is disabled in Kconfig.
> 
> IMHO, such a trick hidden in a macro could cause more confusion
> than good.

Yeah... I wasn't really sure about this either, I don't disagree that it's
marginally evil.
 
> 
> > +		(drv)->cons = allocate_console(ops, name, flags, idx, drv); \
> > +		__retval = (drv)->cons ? 0 : -ENOMEM;			    \
> > +	}								    \
> > +	__retval;							    \
> > +})
> > +
> > +#define uart_allocate_console_dfl(drv, ops, name, kcfg) \
> > +	uart_allocate_console(drv, ops, name, CON_PRINTBUFFER, -1, kcfg)
> > +
> > +#define uart_put_console(drv) put_console((drv)->cons)
> > +
> >  /*
> >   * Power Management
> >   */
> > diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> > index 2e0eb89f046c..67e1e993ab80 100644
> > --- a/kernel/printk/printk.c
> > +++ b/kernel/printk/printk.c
> > @@ -108,6 +108,8 @@ enum devkmsg_log_masks {
> >  
> >  static unsigned int __read_mostly devkmsg_log = DEVKMSG_LOG_MASK_DEFAULT;
> >  
> > +static int printk_late_done;
> > +
> >  static int __control_devkmsg(char *str)
> >  {
> >  	if (!str)
> > @@ -1731,7 +1733,7 @@ static void call_console_drivers(const char *ext_text, size_t ext_len,
> >  			continue;
> >  		if (!(con->flags & CON_ENABLED))
> >  			continue;
> > -		if (!con->write)
> > +		if (!con->ops->write)
> >  			continue;
> >  		if (!cpu_online(smp_processor_id()) &&
> >  		    !(con->flags & CON_ANYTIME))
> > @@ -1739,9 +1741,9 @@ static void call_console_drivers(const char *ext_text, size_t ext_len,
> >  		if (suppress_message_printing(level, con))
> >  			continue;
> >  		if (con->flags & CON_EXTENDED)
> > -			con->write(con, ext_text, ext_len);
> > +			con->ops->write(con, ext_text, ext_len);
> >  		else
> > -			con->write(con, text, len);
> > +			con->ops->write(con, text, len);
> >  	}
> >  }
> >  
> > @@ -2052,7 +2054,7 @@ asmlinkage __visible void early_printk(const char *fmt, ...)
> >  	n = vscnprintf(buf, sizeof(buf), fmt, ap);
> >  	va_end(ap);
> >  
> > -	early_console->write(early_console, buf, n);
> > +	early_console->ops->write(early_console, buf, n);
> >  }
> >  #endif
> >  
> > @@ -2481,8 +2483,8 @@ void console_unblank(void)
> >  	console_locked = 1;
> >  	console_may_schedule = 0;
> >  	for_each_console(c)
> > -		if ((c->flags & CON_ENABLED) && c->unblank)
> > -			c->unblank();
> > +		if ((c->flags & CON_ENABLED) && c->ops->unblank)
> > +			c->ops->unblank();
> >  	console_unlock();
> >  }
> >  
> > @@ -2515,9 +2517,9 @@ struct tty_driver *console_device(int *index)
> >  
> >  	console_lock();
> >  	for_each_console(c) {
> > -		if (!c->device)
> > +		if (!c->ops->device)
> >  			continue;
> > -		driver = c->device(c, index);
> > +		driver = c->ops->device(c, index);
> >  		if (driver)
> >  			break;
> >  	}
> > @@ -2558,6 +2560,68 @@ static int __init keep_bootcon_setup(char *str)
> >  
> >  early_param("keep_bootcon", keep_bootcon_setup);
> >  
> > +static struct bus_type console_subsys = {
> > +	.name = "console",
> > +};
> > +
> > +static void console_release(struct device *dev)
> > +{
> > +	struct console *con = container_of(dev, struct console, dev);
> > +
> > +	if (WARN(con->is_static, "Freeing static early console!\n"))
> > +		return;
> > +
> > +	if (WARN(con->flags & CON_ENABLED, "Freeing running console!\n"))
> > +		return;
> > +
> > +	pr_info("Freeing console %s\n", con->name);
> > +	kfree(con);
> > +}
> > +
> > +static void console_init_device(struct console *con)
> > +{
> > +	device_initialize(&con->dev);
> > +	dev_set_name(&con->dev, "%s", con->name);
> > +	con->dev.release = console_release;
> > +}
> > +
> > +static void console_register_device(struct console *new)
> > +{
> > +	/*
> > +	 * We might be called very early from register_console(): in that case,
> > +	 * printk_late_init() will take care of this later.
> > +	 */
> > +	if (!printk_late_done)
> > +		return;
> > +
> > +	if (new->is_static)
> > +		console_init_device(new);
> > +
> > +	new->dev.bus = &console_subsys;
> > +	WARN_ON(device_add(&new->dev));
> > +}
> > +
> > +struct console *allocate_console(const struct console_operations *ops,
> > +				 const char *name, short flags, short index,
> > +				 void *data)
> > +{
> > +	struct console *new;
> > +
> > +	new = kzalloc(sizeof(*new), GFP_KERNEL);
> 
> I have just realized that page_alloc_init() is called before
> parse_early_param(). Therefore we should be able to use
> memblock_alloc() for early consoles and reduce problems
> with static structures.

Ah nice, I'll look into that. This would all be a lot nicer without
the special cases.
 
> 
> > +	if (!new)
> > +		return NULL;
> > +
> > +	new->ops = ops;
> > +	strscpy(new->name, name, sizeof(new->name));
> > +	new->flags = flags;
> > +	new->index = index;
> > +	new->data = data;
> > +
> > +	console_init_device(new);
> 
> This is a side effect that I would not expect from a function called
> alloc*(). I would rename:
> 
> s/allocate_console/create_console/  or
> s/allocate_console/init_console/

Makes sense.

> 
> > +	return new;
> > +}
> > +EXPORT_SYMBOL_GPL(allocate_console);
> > +
> >  /*
> >   * The console driver calls this routine during kernel initialization
> >   * to register the console printing procedure with printk() and to
> > @@ -2622,10 +2686,10 @@ void register_console(struct console *newcon)
> >  	if (!has_preferred) {
> >  		if (newcon->index < 0)
> >  			newcon->index = 0;
> > -		if (newcon->setup == NULL ||
> > -		    newcon->setup(newcon, NULL) == 0) {
> > +		if (newcon->ops->setup == NULL ||
> > +		    newcon->ops->setup(newcon, NULL) == 0) {
> >  			newcon->flags |= CON_ENABLED;
> > -			if (newcon->device) {
> > +			if (newcon->ops->device) {
> 
> There might be confusion why we need two devices (con->dev
> and con->ops->device).
> 
> I would rename con->ops->device to con->ops->tty_dev.

Makes sense.

> 
> >  				newcon->flags |= CON_CONSDEV;
> >  				has_preferred = true;
> >  			}
> > @@ -2639,8 +2703,8 @@ void register_console(struct console *newcon)
> >  	for (i = 0, c = console_cmdline;
> >  	     i < MAX_CMDLINECONSOLES && c->name[0];
> >  	     i++, c++) {
> > -		if (!newcon->match ||
> > -		    newcon->match(newcon, c->name, c->index, c->options) != 0) {
> > +		if (!newcon->ops->match ||
> > +		    newcon->ops->match(newcon, c->name, c->index, c->options) != 0) {
> >  			/* default matching */
> >  			BUILD_BUG_ON(sizeof(c->name) != sizeof(newcon->name));
> >  			if (strcmp(c->name, newcon->name) != 0)
> > @@ -2660,8 +2724,8 @@ void register_console(struct console *newcon)
> >  			if (_braille_register_console(newcon, c))
> >  				return;
> >  
> > -			if (newcon->setup &&
> > -			    newcon->setup(newcon, c->options) != 0)
> > +			if (newcon->ops->setup &&
> > +			    newcon->ops->setup(newcon, c->options) != 0)
> >  				break;
> >  		}
> >  
> > @@ -2706,6 +2770,8 @@ void register_console(struct console *newcon)
> >  		console_drivers->next = newcon;
> >  	}
> >  
> > +	get_console(newcon);
> > +
> >  	if (newcon->flags & CON_EXTENDED)
> >  		nr_ext_console_drivers++;
> >  
> > @@ -2730,6 +2796,7 @@ void register_console(struct console *newcon)
> >  		exclusive_console_stop_seq = console_seq;
> >  		logbuf_unlock_irqrestore(flags);
> >  	}
> > +	console_register_device(newcon);
> 
> This calls console_init_device() for the statically defined
> early consoles. I guess that it would invalidate the above
> get_console().

The get_console() macro checks for ->is_static and is a NOP if it's
set, which is definitely confusing. 
 
> Also it is not symmetric with unregister_console(). We add it
> to the console_subsys here and did not remove it when
> the console is unregistered.

We do a put() in the unregister path, somebody could hold a reference
through sysfs so we can't just remove it. Or am I misunderstanding?

> IMHO, this might be done already in allocate_console().
> And eventually in printk_late_init() for consoles that
> are allocated earlier.
> 
> I am not familiar with the device-related sysfs API.
> But I see that device_initialize() and device_add()
> can be done by device_register(). The separate
> calls are needed only when a special reference
> handling is needed. Are we special?

We're special: the console initcalls run before the device core is
initialized, initialize() lets us use the refcount.

That said, I'm hopeful your suggestion about using memblock_alloc()
will make some of this confusion unnecessary, it's definitely...
confusing.

> Anyway, please, try to describe the expected workflow
> in the commit description:
> 
>     + where the structure should get allocated
>     + when it is added to the sysfs hierarchy
>     + what happens when the console gets registered
>       and unregistered
>     + when it is removed from the sysfs hierarchy
>     + when the structure is released/freed
> 
>     + What is needed to get the console registered
>       and unregistered repeatedly on a running system
> 
> Finally, we might want to show state of the CON_ENABLED flag
> in sysfs to distinguish the currently used consoles.

Makes sense.
 
> 
> >  	console_unlock();
> >  	console_sysfs_notify();
> 
> Thanks a lot for working on this. I know that it is not easy.
> But it is really appreciated.

Thanks :)

--Calvin

> Best Regards,
> Petr

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