printk: nbcon: Add detailed doc for write_atomic()

The write_atomic() callback has special requirements and is
allowed to use special helper functions. Provide detailed
documentation of the callback so that a developer has a
chance of implementing it correctly.
Signed-off-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20240820063001.36405-8-john.ogness@linutronix.deSigned-off-by: Petr Mladek <pmladek@suse.com>

include/linux/console.h

@@ -303,7 +303,7 @@ struct nbcon_write_context {
 /**
  * struct console - The console descriptor structure
  * @name:		The name of the console driver
- * @write:		Write callback to output messages (Optional)
+ * @write:		Legacy write callback to output messages (Optional)
  * @read:		Read callback for console input (Optional)
  * @device:		The underlying TTY device driver (Optional)
  * @unblank:		Callback to unblank the console (Optional)
@@ -320,7 +320,6 @@ struct nbcon_write_context {
  * @data:		Driver private data
  * @node:		hlist node for the console list
  *
- * @write_atomic:	Write callback for atomic context
  * @nbcon_state:	State for nbcon consoles
  * @nbcon_seq:		Sequence number of the next record for nbcon to print
  * @pbufs:		Pointer to nbcon private buffer
@@ -345,8 +344,34 @@ struct console {
 	struct hlist_node	node;
 
 	/* nbcon console specific members */
-	void			(*write_atomic)(struct console *con,
-						struct nbcon_write_context *wctxt);
+
+	/**
+	 * @write_atomic:
+	 *
+	 * NBCON callback to write out text in any context. (Optional)
+	 *
+	 * This callback is called with the console already acquired. However,
+	 * a higher priority context is allowed to take it over by default.
+	 *
+	 * The callback must call nbcon_enter_unsafe() and nbcon_exit_unsafe()
+	 * around any code where the takeover is not safe, for example, when
+	 * manipulating the serial port registers.
+	 *
+	 * nbcon_enter_unsafe() will fail if the context has lost the console
+	 * ownership in the meantime. In this case, the callback is no longer
+	 * allowed to go forward. It must back out immediately and carefully.
+	 * The buffer content is also no longer trusted since it no longer
+	 * belongs to the context.
+	 *
+	 * The callback should allow the takeover whenever it is safe. It
+	 * increases the chance to see messages when the system is in trouble.
+	 *
+	 * The callback can be called from any context (including NMI).
+	 * Therefore it must avoid usage of any locking and instead rely
+	 * on the console ownership for synchronization.
+	 */
+	void (*write_atomic)(struct console *con, struct nbcon_write_context *wctxt);
+
 	atomic_t		__private nbcon_state;
 	atomic_long_t		__private nbcon_seq;
 	struct printk_buffers	*pbufs;