printk: nbcon: Add function for printers to reacquire ownership

Since ownership can be lost at any time due to handover or
takeover, a printing context _must_ be prepared to back out
immediately and carefully. However, there are scenarios where
the printing context must reacquire ownership in order to
finalize or revert hardware changes.

One such example is when interrupts are disabled during
printing. No other context will automagically re-enable the
interrupts. For this case, the disabling context _must_
reacquire nbcon ownership so that it can re-enable the
interrupts.

Provide nbcon_reacquire_nobuf() for exactly this purpose. It
allows a printing context to reacquire ownership using the same
priority as its previous ownership.

Note that after a successful reacquire the printing context
will have no output buffer because that has been lost. This
function cannot be used to resume printing.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20240904120536.115780-2-john.ogness@linutronix.de
Signed-off-by: Petr Mladek <pmladek@suse.com>

1 files changed, 6 insertions, 0 deletions

diff --git a/include/linux/console.h b/include/linux/console.h
index 9a13f91b0c43..88050d30a9cc 100644
--- a/include/linux/console.h
+++ b/include/linux/console.h
@@ -366,6 +366,10 @@ struct console {
 	 *
 	 * The callback should allow the takeover whenever it is safe. It
 	 * increases the chance to see messages when the system is in trouble.
+	 * If the driver must reacquire ownership in order to finalize or
+	 * revert hardware changes, nbcon_reacquire_nobuf() can be used.
+	 * However, on reacquire the buffer content is no longer available. A
+	 * reacquire cannot be used to resume printing.
 	 *
 	 * The callback can be called from any context (including NMI).
 	 * Therefore it must avoid usage of any locking and instead rely
@@ -558,12 +562,14 @@ extern void nbcon_cpu_emergency_exit(void);
 extern bool nbcon_can_proceed(struct nbcon_write_context *wctxt);
 extern bool nbcon_enter_unsafe(struct nbcon_write_context *wctxt);
 extern bool nbcon_exit_unsafe(struct nbcon_write_context *wctxt);
+extern void nbcon_reacquire_nobuf(struct nbcon_write_context *wctxt);
 #else
 static inline void nbcon_cpu_emergency_enter(void) { }
 static inline void nbcon_cpu_emergency_exit(void) { }
 static inline bool nbcon_can_proceed(struct nbcon_write_context *wctxt) { return false; }
 static inline bool nbcon_enter_unsafe(struct nbcon_write_context *wctxt) { return false; }
 static inline bool nbcon_exit_unsafe(struct nbcon_write_context *wctxt) { return false; }
+static inline void nbcon_reacquire_nobuf(struct nbcon_write_context *wctxt) { }
 #endif
 
 extern int console_set_on_cmdline;