patches/0018-console-add-write_atomic-interface.patch

From: John Ogness <john.ogness@linutronix.de>
Date: Tue, 12 Feb 2019 15:29:56 +0100
Subject: [PATCH 18/25] console: add write_atomic interface

Add a write_atomic callback to the console. This is an optional
function for console drivers. The function must be atomic (including
NMI safe) for writing to the console.

Console drivers must still implement the write callback. The
write_atomic callback will only be used for emergency messages.

Creating an NMI safe write_atomic that must synchronize with write
requires a careful implementation of the console driver. To aid with
the implementation, a set of console_atomic_* functions are provided:

    void console_atomic_lock(unsigned int *flags);
    void console_atomic_unlock(unsigned int flags);

These functions synchronize using the processor-reentrant cpu lock of
the printk buffer.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
 include/linux/console.h |    4 ++++
 kernel/printk/printk.c  |   12 ++++++++++++
 2 files changed, 16 insertions(+)

--- a/include/linux/console.h
+++ b/include/linux/console.h
@@ -144,6 +144,7 @@ static inline int con_debug_leave(void)
 struct console {
 	char	name[16];
 	void	(*write)(struct console *, const char *, unsigned);
+	void	(*write_atomic)(struct console *, const char *, unsigned);
 	int	(*read)(struct console *, char *, unsigned);
 	struct tty_driver *(*device)(struct console *, int *);
 	void	(*unblank)(void);
@@ -234,4 +235,7 @@ extern void console_init(void);
 void dummycon_register_output_notifier(struct notifier_block *nb);
 void dummycon_unregister_output_notifier(struct notifier_block *nb);
 
+extern void console_atomic_lock(unsigned int *flags);
+extern void console_atomic_unlock(unsigned int flags);
+
 #endif /* _LINUX_CONSOLE_H */
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -3045,3 +3045,15 @@ void kmsg_dump_rewind(struct kmsg_dumper
 }
 EXPORT_SYMBOL_GPL(kmsg_dump_rewind);
 #endif
+
+void console_atomic_lock(unsigned int *flags)
+{
+	prb_lock(&printk_cpulock, flags);
+}
+EXPORT_SYMBOL(console_atomic_lock);
+
+void console_atomic_unlock(unsigned int flags)
+{
+	prb_unlock(&printk_cpulock, flags);
+}
+EXPORT_SYMBOL(console_atomic_unlock);