Let's add update_preferred_console() for driver subsystems to call during
init when the console is ready, and it's character device name is known.
For now, we use it only for the serial layer to allow console=DEVNAME:0.0
style hardware based addressing for consoles.

The earlier attempt on doing this caused a regression with the kernel
command line console order as it added calling __add_preferred_console()
again later on during init. A better approach was suggested by Petr where
we add the deferred console to the console_cmdline[] and update it later
on when the console is ready.

Suggested-by: Petr Mladek <pmladek@suse.com>
Co-developed-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Tony Lindgren <tony.lindgren@linux.intel.com>
---
 drivers/tty/serial/serial_base_bus.c |  8 ++-
 include/linux/printk.h               |  2 +
 kernel/printk/console_cmdline.h      |  1 +
 kernel/printk/printk.c               | 77 +++++++++++++++++++++++-----
 4 files changed, 73 insertions(+), 15 deletions(-)

diff --git a/drivers/tty/serial/serial_base_bus.c b/drivers/tty/serial/serial_base_bus.c
index 5ebacb982f9e..a34f55ef6f37 100644
--- a/drivers/tty/serial/serial_base_bus.c
+++ b/drivers/tty/serial/serial_base_bus.c
@@ -210,7 +210,13 @@ void serial_base_port_device_remove(struct serial_port_device *port_dev)
 static int serial_base_add_one_prefcon(const char *match, const char *dev_name,
 				       int port_id)
 {
-	return 0;
+	int ret;
+
+	ret = update_preferred_console(match, dev_name, port_id);
+	if (ret == -ENOENT)
+		return 0;
+
+	return ret;
 }
 
 #ifdef __sparc__
diff --git a/include/linux/printk.h b/include/linux/printk.h
index 65c5184470f1..fda935280e33 100644
--- a/include/linux/printk.h
+++ b/include/linux/printk.h
@@ -60,6 +60,8 @@ static inline const char *printk_skip_headers(const char *buffer)
 #define CONSOLE_LOGLEVEL_DEFAULT CONFIG_CONSOLE_LOGLEVEL_DEFAULT
 #define CONSOLE_LOGLEVEL_QUIET	 CONFIG_CONSOLE_LOGLEVEL_QUIET
 
+int update_preferred_console(const char *match, const char *name, const short idx);
+
 extern int console_printk[];
 
 #define console_loglevel (console_printk[0])
diff --git a/kernel/printk/console_cmdline.h b/kernel/printk/console_cmdline.h
index 3ca74ad391d6..0ab573b6d4dc 100644
--- a/kernel/printk/console_cmdline.h
+++ b/kernel/printk/console_cmdline.h
@@ -6,6 +6,7 @@ struct console_cmdline
 {
 	char	name[16];			/* Name of the driver	    */
 	int	index;				/* Minor dev. to use	    */
+	char	devname[32];			/* DEVNAME:0.0 style device name */
 	bool	user_specified;			/* Specified by command line vs. platform */
 	char	*options;			/* Options for the driver   */
 #ifdef CONFIG_A11Y_BRAILLE_CONSOLE
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index dddb15f48d59..0e2e442593bd 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -2429,18 +2429,23 @@ static void set_user_specified(struct console_cmdline *c, bool user_specified)
 	console_set_on_cmdline = 1;
 }
 
-static int __add_preferred_console(const char *name, const short idx, char *options,
+static int __add_preferred_console(const char *name, const short idx,
+				   const char *devname, char *options,
 				   char *brl_options, bool user_specified)
 {
 	struct console_cmdline *c;
 	int i;
 
+	if (!name && !devname)
+		return -EINVAL;
+
 	/*
 	 * We use a signed short index for struct console for device drivers to
 	 * indicate a not yet assigned index or port. However, a negative index
-	 * value is not valid for preferred console.
+	 * value is not valid when the console name and index are defined on
+	 * the command line.
 	 */
-	if (idx < 0)
+	if (name && idx < 0)
 		return -EINVAL;
 
 	/*
@@ -2448,9 +2453,10 @@ static int __add_preferred_console(const char *name, const short idx, char *opti
 	 *	if we have a slot free.
 	 */
 	for (i = 0, c = console_cmdline;
-	     i < MAX_CMDLINECONSOLES && c->name[0];
+	     i < MAX_CMDLINECONSOLES && (c->name[0] || c->devname[0]);
 	     i++, c++) {
-		if (strcmp(c->name, name) == 0 && c->index == idx) {
+		if ((name && strcmp(c->name, name) == 0 && c->index == idx) ||
+		    (devname && strcmp(c->devname, devname) == 0)) {
 			if (!brl_options)
 				preferred_console = i;
 			set_user_specified(c, user_specified);
@@ -2461,7 +2467,10 @@ static int __add_preferred_console(const char *name, const short idx, char *opti
 		return -E2BIG;
 	if (!brl_options)
 		preferred_console = i;
-	strscpy(c->name, name, sizeof(c->name));
+	if (name)
+		strscpy(c->name, name);
+	if (devname)
+		strscpy(c->devname, devname);
 	c->options = options;
 	set_user_specified(c, user_specified);
 	braille_set_options(c, brl_options);
@@ -2486,8 +2495,8 @@ __setup("console_msg_format=", console_msg_format_setup);
  */
 static int __init console_setup(char *str)
 {
-	char buf[sizeof(console_cmdline[0].name) + 4]; /* 4 for "ttyS" */
-	char *s, *options, *brl_options = NULL;
+	char buf[sizeof(console_cmdline[0].devname)]; /* name with "ttyS" prefix or devname */
+	char *s, *options, *brl_options = NULL, *chardev = NULL, *devname = NULL;
 	int idx;
 
 	/*
@@ -2496,17 +2505,23 @@ static int __init console_setup(char *str)
 	 * for exactly this purpose.
 	 */
 	if (str[0] == 0 || strcmp(str, "null") == 0) {
-		__add_preferred_console("ttynull", 0, NULL, NULL, true);
+		__add_preferred_console("ttynull", 0, NULL, NULL, NULL, true);
 		return 1;
 	}
 
 	if (_braille_console_setup(&str, &brl_options))
 		return 1;
 
+	/* For a DEVNAME:0.0 style console the character device is unknown early */
+	if (strchr(str, ':'))
+		devname = buf;
+	else
+		chardev = buf;
+
 	/*
 	 * Decode str into name, index, options.
 	 */
-	if (isdigit(str[0]))
+	if (chardev && isdigit(str[0]))
 		scnprintf(buf, sizeof(buf), "ttyS%s", str);
 	else
 		strscpy(buf, str);
@@ -2523,12 +2538,12 @@ static int __init console_setup(char *str)
 #endif
 
 	for (s = buf; *s; s++)
-		if (isdigit(*s) || *s == ',')
+		if ((chardev && isdigit(*s)) || *s == ',')
 			break;
 	idx = simple_strtoul(s, NULL, 10);
 	*s = 0;
 
-	__add_preferred_console(buf, idx, options, brl_options, true);
+	__add_preferred_console(chardev, idx, devname, options, brl_options, true);
 	return 1;
 }
 __setup("console=", console_setup);
@@ -2548,7 +2563,38 @@ __setup("console=", console_setup);
  */
 int add_preferred_console(const char *name, const short idx, char *options)
 {
-	return __add_preferred_console(name, idx, options, NULL, false);
+	return __add_preferred_console(name, idx, NULL, options, NULL, false);
+}
+
+/**
+ * update_preferred_console - Updates a preferred console if a match is found
+ * @devname: Expected console on kernel command line, such as console=DEVNAME:0.0
+ * @name: Name of the console character device to add such as ttyS
+ * @idx: Index for the console
+ *
+ * Allows driver subsystems to update a console after translating the command
+ * line name to the character device name used for the console.
+ *
+ * Return: 0 on success, negative error code on failure.
+ */
+int update_preferred_console(const char *devname, const char *name, const short idx)
+{
+	struct console_cmdline *c = console_cmdline;
+	int i;
+
+	if (!devname || !strlen(devname) || !name || !strlen(name) || idx < 0)
+		return -EINVAL;
+
+	for (i = 0; i < MAX_CMDLINECONSOLES && (c->name[0] || c->devname[0]);
+	     i++, c++) {
+		if (!strcmp(devname, c->devname)) {
+			strscpy(c->name, name);
+			c->index = idx;
+			return 0;
+		}
+	}
+
+	return -ENOENT;
 }
 
 bool console_suspend_enabled = true;
@@ -3318,8 +3364,11 @@ static int try_enable_preferred_console(struct console *newcon,
 	int i, err;
 
 	for (i = 0, c = console_cmdline;
-	     i < MAX_CMDLINECONSOLES && c->name[0];
+	     i < MAX_CMDLINECONSOLES && (c->name[0] || c->devname[0]);
 	     i++, c++) {
+		/* Console not yet initialized? */
+		if (!c->name[0])
+			continue;
 		if (c->user_specified != user_specified)
 			continue;
 		if (!newcon->match ||
-- 
2.45.2

On Thu 2024-06-13 15:51:08, Tony Lindgren wrote:
> Let's add update_preferred_console() for driver subsystems to call during
> init when the console is ready, and it's character device name is known.
> For now, we use it only for the serial layer to allow console=DEVNAME:0.0
> style hardware based addressing for consoles.
> 
> The earlier attempt on doing this caused a regression with the kernel
> command line console order as it added calling __add_preferred_console()
> again later on during init. A better approach was suggested by Petr where
> we add the deferred console to the console_cmdline[] and update it later
> on when the console is ready.

The patch seems to work well. And I am surprised that it is so small ;-)
I have some rather cosmetic comments.

> diff --git a/drivers/tty/serial/serial_base_bus.c b/drivers/tty/serial/serial_base_bus.c
> index 5ebacb982f9e..a34f55ef6f37 100644
> --- a/drivers/tty/serial/serial_base_bus.c
> +++ b/drivers/tty/serial/serial_base_bus.c
> @@ -210,7 +210,13 @@ void serial_base_port_device_remove(struct serial_port_device *port_dev)
>  static int serial_base_add_one_prefcon(const char *match, const char *dev_name,
>  				       int port_id)

I would suggest to rename also functions on the serial_base side.
The function is not adding prefcon. It is doing some match_and_update
job.

>  {
> -	return 0;
> +	int ret;
> +
> +	ret = update_preferred_console(match, dev_name, port_id);
> +	if (ret == -ENOENT)
> +		return 0;
> +
> +	return ret;
>  }
>  
>  #ifdef __sparc__
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -2486,8 +2495,8 @@ __setup("console_msg_format=", console_msg_format_setup);
>   */
>  static int __init console_setup(char *str)
>  {
> -	char buf[sizeof(console_cmdline[0].name) + 4]; /* 4 for "ttyS" */
> -	char *s, *options, *brl_options = NULL;

I would add

	static_assert(sizeof(console_cmdline[0].devname) >= sizeof(console_cmdline[0].name));

> +	char buf[sizeof(console_cmdline[0].devname)]; /* name with "ttyS" prefix or devname */
> +	char *s, *options, *brl_options = NULL, *chardev = NULL, *devname = NULL;

The name "chardev" sounds as generic as "devname". I would use one of

   + "name" like the parameter in __add_preferred_console
   + "ttyname" as it is mostly used for "tty*" console names
   + "conname" like a name in struct console.

Also please split the variables per-line so that future diff's are
easier to follow. Something like:

	static_assert(sizeof(console_cmdline[0].devname) >= sizeof(console_cmdline[0].name));
	char buf[sizeof(console_cmdline[0].devname)];
	char *brl_options = NULL;
	char *ttyname = NULL;
	char *devname = NULL;
	char *options;
	char *s;
	int idx;

>  	int idx;
>  
>  	/*
> @@ -2496,17 +2505,23 @@ static int __init console_setup(char *str)
>  	 * for exactly this purpose.
>  	 */
>  	if (str[0] == 0 || strcmp(str, "null") == 0) {
> -		__add_preferred_console("ttynull", 0, NULL, NULL, true);
> +		__add_preferred_console("ttynull", 0, NULL, NULL, NULL, true);
>  		return 1;
>  	}
>  
>  	if (_braille_console_setup(&str, &brl_options))
>  		return 1;
>  
> +	/* For a DEVNAME:0.0 style console the character device is unknown early */
> +	if (strchr(str, ':'))
> +		devname = buf;
> +	else
> +		chardev = buf;
> +
>  	/*
>  	 * Decode str into name, index, options.
>  	 */
> -	if (isdigit(str[0]))
> +	if (chardev && isdigit(str[0]))
>  		scnprintf(buf, sizeof(buf), "ttyS%s", str);
>  	else
>  		strscpy(buf, str);
> @@ -2523,12 +2538,12 @@ static int __init console_setup(char *str)
>  #endif
>  
>  	for (s = buf; *s; s++)
> -		if (isdigit(*s) || *s == ',')
> +		if ((chardev && isdigit(*s)) || *s == ',')
>  			break;
>  	idx = simple_strtoul(s, NULL, 10);

The @idx value is not really important when @devname is used.
But it still would be more clear to set it to -1.

	/* @idx will get defined when devname matches. */
	if (devname)
		idx=-1;
	else
		idx = simple_strtoul(s, NULL, 10);

>  	*s = 0;
>  
> -	__add_preferred_console(buf, idx, options, brl_options, true);
> +	__add_preferred_console(chardev, idx, devname, options, brl_options, true);
>  	return 1;
>  }
>  __setup("console=", console_setup);
> @@ -2548,7 +2563,38 @@ __setup("console=", console_setup);
>   */
>  int add_preferred_console(const char *name, const short idx, char *options)
>  {
> -	return __add_preferred_console(name, idx, options, NULL, false);
> +	return __add_preferred_console(name, idx, NULL, options, NULL, false);
> +}
> +
> +/**
> + * update_preferred_console - Updates a preferred console if a match is found
> + * @devname: Expected console on kernel command line, such as console=DEVNAME:0.0
> + * @name: Name of the console character device to add such as ttyS
> + * @idx: Index for the console
> + *
> + * Allows driver subsystems to update a console after translating the command
> + * line name to the character device name used for the console.

I am not sure what the above sentence exactly means.
IMHO, the character device is too generic term.

> + *
> + * Return: 0 on success, negative error code on failure.
> + */

My proposal might be kind of naive. Some people might say that
it describes obvious things. But the API is for device driver
users which do not know much about how printk handles
the console command line and the registration.

<proposal>
/**
 * match_devname_and_update_preferred_console - Update a preferred console
 *	when matching devname is found.
 * @devname: DEVNAME:0.0 style device name
 * @name: Name of the corresponding console driver, e.g. "ttyS"
 * @idx: Console index, e.g. port number.
 *
 * The function checks whether a device with the given @devname is
 * preferred via the console=DEVNAME:0.0 command line option.
 * It fills the missing console driver name and console index
 * so that a later register_console() call could find (match)
 * and enable this device.
 *
 * It might be used when a driver subsystem initializes particular
 * devices with already known DEVNAME:0.0 style names. And it
 * could predict which console driver name and index this device
 * would later get associated with.
 *
 * Return: 0 on success, negative error code on failure.
 */
</proposal>

At least, this is my understanding of how this works.

I do not know the whole history. And maybe I get something wrong.
IMHO, the main problem is that the printk console code
historically uses TTY device names. But we want to register/enable
the consoles ASAP when the HW devices are ready for writing().
It happens before the TTY subsystem gets initialized so
that we could not use the sysfs kobjects for matching
the tty device driver names with HW device driver names.
And we need this kind of hacks.

But maybe I do not have the right picture about the initialization
and the names of the pieces.


> +int update_preferred_console(const char *devname, const char *name, const short idx)
> +{
> +	struct console_cmdline *c = console_cmdline;
> +	int i;
> +
> +	if (!devname || !strlen(devname) || !name || !strlen(name) || idx < 0)
> +		return -EINVAL;
> +
> +	for (i = 0; i < MAX_CMDLINECONSOLES && (c->name[0] || c->devname[0]);
> +	     i++, c++) {
> +		if (!strcmp(devname, c->devname)) {

I would add here:

			pr_info("associate the preferred console \"%s\" with \"%s%d\"\n",
				devname, name, idx);

> +			strscpy(c->name, name);
> +			c->index = idx;
> +			return 0;
> +		}
> +	}
> +
> +	return -ENOENT;
>  }
>  
>  bool console_suspend_enabled = true;

Best Regards,
Petr

On Fri, Jun 14, 2024 at 07:17:20PM +0200, Petr Mladek wrote:
> On Thu 2024-06-13 15:51:08, Tony Lindgren wrote:
> > The earlier attempt on doing this caused a regression with the kernel
> > command line console order as it added calling __add_preferred_console()
> > again later on during init. A better approach was suggested by Petr where
> > we add the deferred console to the console_cmdline[] and update it later
> > on when the console is ready.
> 
> The patch seems to work well. And I am surprised that it is so small ;-)

Your intuition at work :)

> > diff --git a/drivers/tty/serial/serial_base_bus.c b/drivers/tty/serial/serial_base_bus.c
> > index 5ebacb982f9e..a34f55ef6f37 100644
> > --- a/drivers/tty/serial/serial_base_bus.c
> > +++ b/drivers/tty/serial/serial_base_bus.c
> > @@ -210,7 +210,13 @@ void serial_base_port_device_remove(struct serial_port_device *port_dev)
> >  static int serial_base_add_one_prefcon(const char *match, const char *dev_name,
> >  				       int port_id)
> 
> I would suggest to rename also functions on the serial_base side.
> The function is not adding prefcon. It is doing some match_and_update
> job.

OK good idea, I'll do a separate patch for that.

> > --- a/kernel/printk/printk.c
> > +++ b/kernel/printk/printk.c
> > @@ -2486,8 +2495,8 @@ __setup("console_msg_format=", console_msg_format_setup);
> >   */
> >  static int __init console_setup(char *str)
> >  {
> > -	char buf[sizeof(console_cmdline[0].name) + 4]; /* 4 for "ttyS" */
> > -	char *s, *options, *brl_options = NULL;
> 
> I would add
> 
> 	static_assert(sizeof(console_cmdline[0].devname) >= sizeof(console_cmdline[0].name));

That check should still be >= sizeof(console_cmdline[0].name) + 4, right?
For the "number only" consoles we add the "ttyS" prefix.

> The name "chardev" sounds as generic as "devname". I would use one of
> 
>    + "name" like the parameter in __add_preferred_console
>    + "ttyname" as it is mostly used for "tty*" console names
>    + "conname" like a name in struct console.
> 
> Also please split the variables per-line so that future diff's are
> easier to follow. Something like:
> 
> 	static_assert(sizeof(console_cmdline[0].devname) >= sizeof(console_cmdline[0].name));
> 	char buf[sizeof(console_cmdline[0].devname)];
> 	char *brl_options = NULL;
> 	char *ttyname = NULL;
> 	char *devname = NULL;
> 	char *options;
> 	char *s;
> 	int idx;

OK I'll use ttyname like you're suggesting.

> > @@ -2523,12 +2538,12 @@ static int __init console_setup(char *str)
> >  #endif
> >  
> >  	for (s = buf; *s; s++)
> > -		if (isdigit(*s) || *s == ',')
> > +		if ((chardev && isdigit(*s)) || *s == ',')
> >  			break;
> >  	idx = simple_strtoul(s, NULL, 10);
> 
> The @idx value is not really important when @devname is used.
> But it still would be more clear to set it to -1.

OK

> My proposal might be kind of naive. Some people might say that
> it describes obvious things. But the API is for device driver
> users which do not know much about how printk handles
> the console command line and the registration.
> 
> <proposal>
> /**
>  * match_devname_and_update_preferred_console - Update a preferred console
>  *	when matching devname is found.
>  * @devname: DEVNAME:0.0 style device name
>  * @name: Name of the corresponding console driver, e.g. "ttyS"
>  * @idx: Console index, e.g. port number.
>  *
>  * The function checks whether a device with the given @devname is
>  * preferred via the console=DEVNAME:0.0 command line option.
>  * It fills the missing console driver name and console index
>  * so that a later register_console() call could find (match)
>  * and enable this device.
>  *
>  * It might be used when a driver subsystem initializes particular
>  * devices with already known DEVNAME:0.0 style names. And it
>  * could predict which console driver name and index this device
>  * would later get associated with.
>  *
>  * Return: 0 on success, negative error code on failure.
>  */
> </proposal>

That looks good to me.

> At least, this is my understanding of how this works.
> 
> I do not know the whole history. And maybe I get something wrong.
> IMHO, the main problem is that the printk console code
> historically uses TTY device names. But we want to register/enable
> the consoles ASAP when the HW devices are ready for writing().
> It happens before the TTY subsystem gets initialized so
> that we could not use the sysfs kobjects for matching
> the tty device driver names with HW device driver names.
> And we need this kind of hacks.

Yes that's correct.

> > +int update_preferred_console(const char *devname, const char *name, const short idx)
> > +{
> > +	struct console_cmdline *c = console_cmdline;
> > +	int i;
> > +
> > +	if (!devname || !strlen(devname) || !name || !strlen(name) || idx < 0)
> > +		return -EINVAL;
> > +
> > +	for (i = 0; i < MAX_CMDLINECONSOLES && (c->name[0] || c->devname[0]);
> > +	     i++, c++) {
> > +		if (!strcmp(devname, c->devname)) {
> 
> I would add here:
> 
> 			pr_info("associate the preferred console \"%s\" with \"%s%d\"\n",
> 				devname, name, idx);
> 

OK

Regards,

Tony

On Mon 2024-06-17 10:20:57, Tony Lindgren wrote:
> On Fri, Jun 14, 2024 at 07:17:20PM +0200, Petr Mladek wrote:
> > On Thu 2024-06-13 15:51:08, Tony Lindgren wrote:
> > > The earlier attempt on doing this caused a regression with the kernel
> > > command line console order as it added calling __add_preferred_console()
> > > again later on during init. A better approach was suggested by Petr where
> > > we add the deferred console to the console_cmdline[] and update it later
> > > on when the console is ready.
> > 
> > > --- a/kernel/printk/printk.c
> > > +++ b/kernel/printk/printk.c
> > > @@ -2486,8 +2495,8 @@ __setup("console_msg_format=", console_msg_format_setup);
> > >   */
> > >  static int __init console_setup(char *str)
> > >  {
> > > -	char buf[sizeof(console_cmdline[0].name) + 4]; /* 4 for "ttyS" */
> > > -	char *s, *options, *brl_options = NULL;
> > 
> > I would add
> > 
> > 	static_assert(sizeof(console_cmdline[0].devname) >= sizeof(console_cmdline[0].name));
> 
> That check should still be >= sizeof(console_cmdline[0].name) + 4, right?
> For the "number only" consoles we add the "ttyS" prefix.

Right. Great catch!

Best Regards,
Petr