printk: Add non-BKL (nbcon) console basic infrastructure

The current console/printk subsystem is protected by a Big Kernel Lock,
(aka console_lock) which has ill defined semantics and is more or less
stateless. This puts severe limitations on the console subsystem and
makes forced takeover and output in emergency and panic situations a
fragile endeavour that is based on try and pray.

The goal of non-BKL (nbcon) consoles is to break out of the console lock
jail and to provide a new infrastructure that avoids the pitfalls and
also allows console drivers to be gradually converted over.

The proposed infrastructure aims for the following properties:

  - Per console locking instead of global locking
  - Per console state that allows to make informed decisions
  - Stateful handover and takeover

As a first step, state is added to struct console. The per console state
is an atomic_t using a 32bit bit field.

Reserve state bits, which will be populated later in the series. Wire
it up into the console register/unregister functionality.

It was decided to use a bitfield because using a plain u32 with
mask/shift operations resulted in uncomprehensible code.

Co-developed-by: John Ogness <john.ogness@linutronix.de>
Signed-off-by: John Ogness <john.ogness@linutronix.de>
Signed-off-by: Thomas Gleixner (Intel) <tglx@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20230916192007.608398-2-john.ogness@linutronix.de

include/linux/console.h

@@ -156,6 +156,8 @@ static inline int con_debug_leave(void)
  *			/dev/kmesg which requires a larger output buffer.
  * @CON_SUSPENDED:	Indicates if a console is suspended. If true, the
  *			printing callbacks must not be called.
+ * @CON_NBCON:		Console can operate outside of the legacy style console_lock
+ *			constraints.
  */
 enum cons_flags {
 	CON_PRINTBUFFER		= BIT(0),
@@ -166,8 +168,32 @@ enum cons_flags {
 	CON_BRL			= BIT(5),
 	CON_EXTENDED		= BIT(6),
 	CON_SUSPENDED		= BIT(7),
+	CON_NBCON		= BIT(8),
 };
 
+/**
+ * struct nbcon_state - console state for nbcon consoles
+ * @atom:	Compound of the state fields for atomic operations
+ *
+ * To be used for reading and preparing of the value stored in the nbcon
+ * state variable @console::nbcon_state.
+ */
+struct nbcon_state {
+	union {
+		unsigned int	atom;
+		struct {
+		};
+	};
+};
+
+/*
+ * The nbcon_state struct is used to easily create and interpret values that
+ * are stored in the @console::nbcon_state variable. Ensure this struct stays
+ * within the size boundaries of the atomic variable's underlying type in
+ * order to avoid any accidental truncation.
+ */
+static_assert(sizeof(struct nbcon_state) <= sizeof(int));
+
 /**
  * struct console - The console descriptor structure
  * @name:		The name of the console driver
@@ -187,6 +213,8 @@ enum cons_flags {
  * @dropped:		Number of unreported dropped ringbuffer records
  * @data:		Driver private data
  * @node:		hlist node for the console list
+ *
+ * @nbcon_state:	State for nbcon consoles
  */
 struct console {
 	char			name[16];
@@ -206,6 +234,9 @@ struct console {
 	unsigned long		dropped;
 	void			*data;
 	struct hlist_node	node;
+
+	/* nbcon console specific members */
+	atomic_t		__private nbcon_state;
 };
 
 #ifdef CONFIG_LOCKDEP

kernel/printk/Makefile

@@ -1,6 +1,6 @@
 # SPDX-License-Identifier: GPL-2.0-only
 obj-y	= printk.o
-obj-$(CONFIG_PRINTK)	+= printk_safe.o
+obj-$(CONFIG_PRINTK)	+= printk_safe.o nbcon.o
 obj-$(CONFIG_A11Y_BRAILLE_CONSOLE)	+= braille.o
 obj-$(CONFIG_PRINTK_INDEX)	+= index.o
 

kernel/printk/internal.h

@@ -3,6 +3,7 @@
  * internal.h - printk internal definitions
  */
 #include <linux/percpu.h>
+#include <linux/console.h>
 
 #if defined(CONFIG_PRINTK) && defined(CONFIG_SYSCTL)
 void __init printk_sysctl_init(void);
@@ -61,6 +62,10 @@ void defer_console_output(void);
 
 u16 printk_parse_prefix(const char *text, int *level,
 			enum printk_info_flags *flags);
+
+void nbcon_init(struct console *con);
+void nbcon_cleanup(struct console *con);
+
 #else
 
 #define PRINTK_PREFIX_MAX	0
@@ -76,6 +81,9 @@ u16 printk_parse_prefix(const char *text, int *level,
 #define printk_safe_exit_irqrestore(flags) local_irq_restore(flags)
 
 static inline bool printk_percpu_data_ready(void) { return false; }
+static inline void nbcon_init(struct console *con) { }
+static inline void nbcon_cleanup(struct console *con) { }
+
 #endif /* CONFIG_PRINTK */
 
 /**

kernel/printk/nbcon.c

@@ -0,0 +1,70 @@
+// SPDX-License-Identifier: GPL-2.0-only
+// Copyright (C) 2022 Linutronix GmbH, John Ogness
+// Copyright (C) 2022 Intel, Thomas Gleixner
+
+#include <linux/kernel.h>
+#include <linux/console.h>
+#include "internal.h"
+/*
+ * Printk console printing implementation for consoles which does not depend
+ * on the legacy style console_lock mechanism.
+ */
+
+/**
+ * nbcon_state_set - Helper function to set the console state
+ * @con:	Console to update
+ * @new:	The new state to write
+ *
+ * Only to be used when the console is not yet or no longer visible in the
+ * system. Otherwise use nbcon_state_try_cmpxchg().
+ */
+static inline void nbcon_state_set(struct console *con, struct nbcon_state *new)
+{
+	atomic_set(&ACCESS_PRIVATE(con, nbcon_state), new->atom);
+}
+
+/**
+ * nbcon_state_read - Helper function to read the console state
+ * @con:	Console to read
+ * @state:	The state to store the result
+ */
+static inline void nbcon_state_read(struct console *con, struct nbcon_state *state)
+{
+	state->atom = atomic_read(&ACCESS_PRIVATE(con, nbcon_state));
+}
+
+/**
+ * nbcon_state_try_cmpxchg() - Helper function for atomic_try_cmpxchg() on console state
+ * @con:	Console to update
+ * @cur:	Old/expected state
+ * @new:	New state
+ *
+ * Return: True on success. False on fail and @cur is updated.
+ */
+static inline bool nbcon_state_try_cmpxchg(struct console *con, struct nbcon_state *cur,
+					   struct nbcon_state *new)
+{
+	return atomic_try_cmpxchg(&ACCESS_PRIVATE(con, nbcon_state), &cur->atom, new->atom);
+}
+
+/**
+ * nbcon_init - Initialize the nbcon console specific data
+ * @con:	Console to initialize
+ */
+void nbcon_init(struct console *con)
+{
+	struct nbcon_state state = { };
+
+	nbcon_state_set(con, &state);
+}
+
+/**
+ * nbcon_cleanup - Cleanup the nbcon console specific data
+ * @con:	Console to cleanup
+ */
+void nbcon_cleanup(struct console *con)
+{
+	struct nbcon_state state = { };
+
+	nbcon_state_set(con, &state);
+}

kernel/printk/printk.c

@@ -3326,9 +3326,10 @@ static void try_enable_default_console(struct console *newcon)
 		newcon->flags |= CON_CONSDEV;
 }
 
-#define con_printk(lvl, con, fmt, ...)			\
-	printk(lvl pr_fmt("%sconsole [%s%d] " fmt),	\
-	       (con->flags & CON_BOOT) ? "boot" : "",	\
+#define con_printk(lvl, con, fmt, ...)				\
+	printk(lvl pr_fmt("%s%sconsole [%s%d] " fmt),		\
+	       (con->flags & CON_NBCON) ? "" : "legacy ",	\
+	       (con->flags & CON_BOOT) ? "boot" : "",		\
 	       con->name, con->index, ##__VA_ARGS__)
 
 static void console_init_seq(struct console *newcon, bool bootcon_registered)
@@ -3488,6 +3489,9 @@ void register_console(struct console *newcon)
 	newcon->dropped = 0;
 	console_init_seq(newcon, bootcon_registered);
 
+	if (newcon->flags & CON_NBCON)
+		nbcon_init(newcon);
+
 	/*
 	 * Put this console in the list - keep the
 	 * preferred driver at the head of the list.
@@ -3579,6 +3583,9 @@ static int unregister_console_locked(struct console *console)
 	 */
 	synchronize_srcu(&console_srcu);
 
+	if (console->flags & CON_NBCON)
+		nbcon_cleanup(console);
+
 	console_sysfs_notify();
 
 	if (console->exit)