Message-ID: <77aa59337507e067f3a4ad7e15375893612bcfa3.1763492585.git.chris@chrisdown.name>
Date: Wed, 19 Nov 2025 03:07:37 +0800
From: Chris Down <chris@...isdown.name>
To: Petr Mladek <pmladek@...e.com>
Cc: linux-kernel@...r.kernel.org,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Sergey Senozhatsky <senozhatsky@...omium.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	John Ogness <john.ogness@...utronix.de>,
	Geert Uytterhoeven <geert@...ux-m68k.org>,
	Tony Lindgren <tony.lindgren@...ux.intel.com>, kernel-team@...com
Subject: [PATCH v7 09/13] printk: Support setting initial console loglevel
 via console= on cmdline

Extend the console= kernel command line parameter to support specifying
per-console loglevels at boot time. This is achieved by introducing a
new loglevel option that can be passed as a loglevel option within a
console= stanza.

For example, this is an example of how one might configure netconsole
devices to print messages with a higher priority than loglevel 3
(KERN_ERR) at startup:

    console=netcon0,loglevel:3

Signed-off-by: Chris Down <chris@...isdown.name>
---
 .../admin-guide/kernel-parameters.txt         |  24 ++--
 Documentation/admin-guide/serial-console.rst  |  37 +++++-
 Documentation/networking/netconsole.rst       |   6 +-
 kernel/printk/console_cmdline.h               |   1 +
 kernel/printk/printk.c                        | 108 +++++++++++++++++-
 5 files changed, 161 insertions(+), 15 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 7b62a99489b8..dec9e29ba8d2 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -819,13 +819,18 @@
 		ttyS<n>[,options]
 		ttyUSB0[,options]
 			Use the specified serial port.  The options are of
-			the form "bbbbpnf", where "bbbb" is the baud rate,
-			"p" is parity ("n", "o", or "e"), "n" is number of
-			bits, and "f" is flow control ("r" for RTS or
-			omit it).  Default is "9600n8".
+			the form "bbbbpnf,extra", where "bbbb" is the baud
+			rate, "p" is parity ("n", "o", or "e"), "n" is
+			number of bits, and "f" is flow control ("r" for RTS
+			or omit it). Default is "9600n8".
 
-			See Documentation/admin-guide/serial-console.rst for more
-			information.  See
+			At present the only extra option is "loglevel" to
+			set the per-console loglevel. For example:
+
+				console=ttyS0,9600n8,loglevel:3
+
+			See Documentation/admin-guide/serial-console.rst for
+			more information.  See
 			Documentation/networking/netconsole.rst for an
 			alternative.
 
@@ -3411,8 +3416,11 @@
 	loglevel=	[KNL,EARLY]
 			All Kernel Messages with a loglevel smaller than the
 			console loglevel will be printed to the console. It can
-			also be changed with klogd or other programs. The
-			loglevels are defined as follows:
+			also be changed with klogd or other programs. Note that
+			this can be overridden per-console, see
+			Documentation/admin-guide/per-console-loglevel.rst.
+
+			The loglevels are defined as follows:
 
 			0 (KERN_EMERG)		system is unusable
 			1 (KERN_ALERT)		action must be taken immediately
diff --git a/Documentation/admin-guide/serial-console.rst b/Documentation/admin-guide/serial-console.rst
index 1609e7479249..b8163e6eb95a 100644
--- a/Documentation/admin-guide/serial-console.rst
+++ b/Documentation/admin-guide/serial-console.rst
@@ -32,6 +32,33 @@ The format of this option is::
 			and F is flow control ('r' for RTS). Default is
 			9600n8. The maximum baudrate is 115200.
 
+			One can also specify the per-console loglevel for this
+			console by providing a loglevel parameter, for example
+			"loglevel:4" to set this console's loglevel to 4. The
+			value provided can be from 1 (LOGLEVEL_ALERT) to 8
+			(LOGLEVEL_DEBUG + 1), and messages below that will be
+			emitted onto the console as they become available.
+
+The available loglevels are defined thus:
+
++---+--------------+-----------------------------------+
+| 0 | KERN_EMERG   | system is unusable                |
++---+--------------+-----------------------------------+
+| 1 | KERN_ALERT   | action must be taken immediately  |
++---+--------------+-----------------------------------+
+| 2 | KERN_CRIT    | critical conditions               |
++---+--------------+-----------------------------------+
+| 3 | KERN_ERR     | error conditions                  |
++---+--------------+-----------------------------------+
+| 4 | KERN_WARNING | warning conditions                |
++---+--------------+-----------------------------------+
+| 5 | KERN_NOTICE  | normal but significant condition  |
++---+--------------+-----------------------------------+
+| 6 | KERN_INFO    | informational                     |
++---+--------------+-----------------------------------+
+| 7 | KERN_DEBUG   | debug-level messages              |
++---+--------------+-----------------------------------+
+
 You can specify multiple console= options on the kernel command line.
 
 The behavior is well defined when each device type is mentioned only once.
@@ -39,11 +66,14 @@ In this case, the output will appear on all requested consoles. And
 the last device will be used when you open ``/dev/console``.
 So, for example::
 
-	console=ttyS1,9600 console=tty0
+	console=ttyS1,9600,loglevel:5 console=tty0
 
 defines that opening ``/dev/console`` will get you the current foreground
-virtual console, and kernel messages will appear on both the VGA
-console and the 2nd serial port (ttyS1 or COM2) at 9600 baud.
+virtual console, and kernel messages will appear on both the VGA console and
+the 2nd serial port (ttyS1 or COM2) at 9600 baud. The optional loglevel "5"
+indicates that this console will emit messages more serious than
+LOGLEVEL_NOTICE (that is, LOGLEVEL_WARNING and below, since more serious
+messages have lower ordering).
 
 The behavior is more complicated when the same device type is defined more
 times. In this case, there are the following two rules:
@@ -145,3 +175,4 @@ Replace the sample values as needed.
    the integration of these patches into m68k, ppc and alpha.
 
 Miquel van Smoorenburg <miquels@...tron.nl>, 11-Jun-2000
+Chris Down <chris@...isdown.name>, 18-Nov-2025
diff --git a/Documentation/networking/netconsole.rst b/Documentation/networking/netconsole.rst
index 5fabcf9367d1..5a7405986934 100644
--- a/Documentation/networking/netconsole.rst
+++ b/Documentation/networking/netconsole.rst
@@ -78,7 +78,11 @@ Built-in netconsole starts immediately after the TCP stack is
 initialized and attempts to bring up the supplied dev at the supplied
 address.
 
-You can also set a loglevel at runtime::
+You can also set a loglevel at boot time on the kernel command line::
+
+  console=netcon0,loglevel:2
+
+This can also be changed at runtime::
 
   $ ls -l /sys/class/console/netcon0/
   total 0
diff --git a/kernel/printk/console_cmdline.h b/kernel/printk/console_cmdline.h
index 0ab573b6d4dc..cb3b99d31d80 100644
--- a/kernel/printk/console_cmdline.h
+++ b/kernel/printk/console_cmdline.h
@@ -7,6 +7,7 @@ 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 */
+	int	level;				/* Log level to use */
 	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 fb54101133d2..a6e1fa924294 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -2660,12 +2660,108 @@ static void set_user_specified(struct console_cmdline *c, bool user_specified)
 	console_set_on_cmdline = 1;
 }
 
+/**
+ * find_and_remove_console_option - Find and remove a named option from console options string
+ * @options: The console options string (will be modified in-place)
+ * @key: The option name to find (e.g., "loglevel")
+ * @val_buf: Buffer to store the option value (if present)
+ * @val_buf_size: Size of @val_buf
+ *
+ * This function searches for a named option in a comma-separated options string
+ * (e.g., "9600n8,loglevel:3,other:value"). If found, it extracts the value
+ * (the part after ':') and removes the entire option from the string.
+ *
+ * The function modifies @options in-place by:
+ * 1. Temporarily null-terminating option names and values during parsing
+ * 2. Restoring separators if the option isn't found
+ * 3. Removing the found option by shifting the remaining string
+ *
+ * Return: true if the option was found and removed, false otherwise
+ */
+static bool find_and_remove_console_option(char *options, const char *key,
+					   char *val_buf, size_t val_buf_size)
+{
+	bool found = false, first = true;
+	char *option, *next = options;
+
+	while ((option = strsep(&next, ","))) {
+		char *value;
+
+		value = strchr(option, ':');
+		if (value)
+			*(value++) = '\0';
+
+		if (strcmp(option, key) == 0) {
+			found = true;
+			if (value) {
+				if (strlen(value) >= val_buf_size) {
+					pr_warn("Can't copy console option value for %s:%s: not enough space (%zu)\n",
+						option, value, val_buf_size);
+					found = false;
+				} else {
+					strscpy(val_buf, value, val_buf_size);
+				}
+			} else
+				*val_buf = '\0';
+		}
+
+		if (found)
+			break;
+
+		if (next)
+			*(next - 1) = ',';
+		if (value)
+			*(value - 1) = ':';
+
+		first = false;
+	}
+
+	if (found) {
+		if (next)
+			memmove(option, next, strlen(next) + 1);
+		else if (first)
+			*option = '\0';
+		else
+			*--option = '\0';
+	}
+
+	return found;
+}
+
+static int find_and_remove_loglevel_option(char *options)
+{
+	char val[16];
+	int loglevel;
+
+	if (!find_and_remove_console_option(options, "loglevel", val,
+					    sizeof(val)))
+		return -ENOENT;
+
+	if (kstrtoint(val, 10, &loglevel)) {
+		pr_warn("Invalid console loglevel, ignoring: %s\n", val);
+		return -EINVAL;
+	}
+
+	/* Reject level 0 (KERN_EMERG) - per-console loglevel must be > 0 */
+	if (loglevel == 0) {
+		pr_warn("Per-console loglevel 0 (KERN_EMERG) is not allowed, ignoring\n");
+		return -ERANGE;
+	}
+
+	if (console_clamp_loglevel(loglevel) != loglevel) {
+		pr_warn("Per-console loglevel out of range, ignoring: %d\n", loglevel);
+		return -ERANGE;
+	}
+
+	return loglevel;
+}
+
 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;
+	int i, ret;
 
 	if (!name && !devname)
 		return -EINVAL;
@@ -2702,6 +2798,13 @@ static int __add_preferred_console(const char *name, const short idx,
 		strscpy(c->name, name);
 	if (devname)
 		strscpy(c->devname, devname);
+
+	ret = find_and_remove_loglevel_option(options);
+	if (ret >= 0)
+		c->level = ret;
+	else
+		c->level = -1;
+
 	c->options = options;
 	set_user_specified(c, user_specified);
 	braille_set_options(c, brl_options);
@@ -3979,8 +4082,7 @@ static int try_enable_preferred_console(struct console *newcon,
 			if (newcon->index < 0)
 				newcon->index = c->index;
 
-			/* TODO: will be configurable in a later patch */
-			newcon->level = LOGLEVEL_DEFAULT;
+			newcon->level = c->level;
 
 			if (_braille_register_console(newcon, c))
 				return 0;
-- 
2.51.2

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