[PATCH printk 0/3] Introduce sync mode

[PATCH printk 0/3] Introduce sync mode

[John Ogness]

Hi,

As proposed in an LKML thread [0], here is a series to introduce a
new console feature to use synchronous printing. The feature is
activated using the keyword "sync" in the console= command line
argument. For example:

   console=ttyS0,115200,sync

Sync mode is only available for nbcon consoles that provide a
safe write_atomic() callback. Specifying it for other consoles
will have no effect other than a log entry that sync mode is not
supported.

Patch 3/3 shamelessly copied an implementation from Chris Down
to parse and update the console options. (Chris's version of 
find_and_remove_console_option() is here [1]). I slightly extended
the function to support valueless-options.

At some point we may want to add a sysfs interface to toggle sync
mode.

John Ogness

[0] https://lore.kernel.org/lkml/87v7aruub1.fsf@jogness.linutronix.de
[1] https://lore.kernel.org/lkml/77aa59337507e067f3a4ad7e15375893612bcfa3.1763492585.git.chris@chrisdown.name

John Ogness (3):
  printk: Introduce console sync mode
  proc: Add console sync support for /proc/consoles
  printk: Support setting console sync mode via console=

 Documentation/admin-guide/serial-console.rst |   8 ++
 fs/proc/consoles.c                           |   1 +
 include/linux/console.h                      |   2 +
 kernel/printk/console_cmdline.h              |   1 +
 kernel/printk/internal.h                     |   2 +
 kernel/printk/nbcon.c                        |  42 ++++++--
 kernel/printk/printk.c                       | 104 +++++++++++++++++++
 7 files changed, 154 insertions(+), 6 deletions(-)


base-commit: 080d60fffa8e0d285871cde8395438006a9b5b0c
-- 
2.47.3

[PATCH printk 3/3] printk: Support setting console sync mode via console=

[John Ogness]

Extend the console= kernel command line argument to support specifying
sync mode at boot time. This is achieved by introducing a new "sync"
option that can be passed within a console= cmdline argument.

For example, assuming the first serial device should be a console using
sync mode:

   console=ttyS0,115200,sync

Signed-off-by: John Ogness <john.ogness@linutronix.de>
Co-authored-by: Chris Down <chris@chrisdown.name>
---
 Documentation/admin-guide/serial-console.rst |  8 ++
 kernel/printk/console_cmdline.h              |  1 +
 kernel/printk/printk.c                       | 79 ++++++++++++++++++++
 3 files changed, 88 insertions(+)

diff --git a/Documentation/admin-guide/serial-console.rst b/Documentation/admin-guide/serial-console.rst
index 1609e7479249f..37cbd9581c53a 100644
--- a/Documentation/admin-guide/serial-console.rst
+++ b/Documentation/admin-guide/serial-console.rst
@@ -32,6 +32,14 @@ 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 extra options (comma-separated)
+			to modify console printing behavior. The "sync" extra
+			option will cause console printing on that console to
+			be synchronous (printed in the context of the printk()
+			caller). This could cause adverse timing side-effects
+			for the system, but can be useful in certain debugging
+			scenarios.
+
 You can specify multiple console= options on the kernel command line.
 
 The behavior is well defined when each device type is mentioned only once.
diff --git a/kernel/printk/console_cmdline.h b/kernel/printk/console_cmdline.h
index 0ab573b6d4dc2..ce4709f7a1b0c 100644
--- a/kernel/printk/console_cmdline.h
+++ b/kernel/printk/console_cmdline.h
@@ -8,6 +8,7 @@ struct console_cmdline
 	int	index;				/* Minor dev. to use	    */
 	char	devname[32];			/* DEVNAME:0.0 style device name */
 	bool	user_specified;			/* Specified by command line vs. platform */
+	bool	sync;				/* Print in sync mode	    */
 	char	*options;			/* Options for the driver   */
 #ifdef CONFIG_A11Y_BRAILLE_CONSOLE
 	char	*brl_options;			/* Options for braille driver */
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index e82e864a4d672..e53ac5554e5eb 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -2567,6 +2567,81 @@ 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,sync"). If found, it extracts the value
+ * (the part after ':') and removes the entire option from the string.
+ *
+ * If an option does not support values, @val_buf should be NULL, in which
+ * case no value part is extracted and any user provided ':' is considered part
+ * of the option key.
+ *
+ * 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 = NULL;
+
+		if (val_buf) {
+			value = strchr(option, ':');
+			if (value)
+				*(value++) = '\0';
+		}
+
+		if (strcmp(option, key) == 0) {
+			found = true;
+			if (value) {
+				if (strlen(value) >= val_buf_size) {
+					pr_warn("Cannot 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 if (val_buf) {
+				*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 __add_preferred_console(const char *name, const short idx,
 				   const char *devname, char *options,
 				   char *brl_options, bool user_specified)
@@ -2609,6 +2684,7 @@ static int __add_preferred_console(const char *name, const short idx,
 		strscpy(c->name, name);
 	if (devname)
 		strscpy(c->devname, devname);
+	c->sync = find_and_remove_console_option(options, "sync", NULL, 0);
 	c->options = options;
 	set_user_specified(c, user_specified);
 	braille_set_options(c, brl_options);
@@ -3932,6 +4008,9 @@ static int try_enable_preferred_console(struct console *newcon,
 			if (_braille_register_console(newcon, c))
 				return 0;
 
+			if (c->sync)
+				newcon->flags |= CON_SYNC;
+
 			err = console_call_setup(newcon, c->options);
 			if (err)
 				return err;
-- 
2.47.3

Re: [PATCH printk 0/3] Introduce sync mode

[John Ogness]

On 2026-07-10, John Ogness <john.ogness@linutronix.de> wrote:
> As proposed in an LKML thread [0], here is a series to introduce a
> new console feature to use synchronous printing.

So the Sashiko review [0] found all kinds of issues and every one of
them are legitimate. In particular, I totally forgot about:

- the console_lock synchronization when boot consoles exist

- the various sites that _rely_ on kthreads printing when
  printk_get_console_flush_type() reports ft.nbcon_offload

I will consider each of the points mentioned by Sashiko and include my
proposed solution in my response. I will need a few days to go through
all of it.

John

[0] https://sashiko.dev/#/patchset/20260710144609.194487-1-john.ogness%40linutronix.de