From: John Ogness <john.ogness@linutronix.de>
Date: Fri, 20 Oct 2023 10:01:58 +0000
Subject: [PATCH 129/134] printk: nbcon: Provide function to reacquire
 ownership
Origin: https://www.kernel.org/pub/linux/kernel/projects/rt/6.6/older/patches-6.6.7-rt18.tar.xz

Contexts may become nbcon owners for various reasons, not just
for printing. Indeed, the port->lock wrapper takes ownership
for anything relating to the hardware.

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

One such example is when interrupts are disabled by a context.
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() for exactly this purpose.

Note that for printing contexts, after a successful reacquire
the context will have no output buffer because that has been
lost. nbcon_reacquire() cannot be used to resume printing.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
 include/linux/console.h |    2 ++
 kernel/printk/nbcon.c   |   32 ++++++++++++++++++++++++++++++++
 2 files changed, 34 insertions(+)

--- a/include/linux/console.h
+++ b/include/linux/console.h
@@ -474,12 +474,14 @@ extern void nbcon_cpu_emergency_exit(voi
 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(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(struct nbcon_write_context *wctxt) { }
 #endif
 
 extern int console_set_on_cmdline;
--- a/kernel/printk/nbcon.c
+++ b/kernel/printk/nbcon.c
@@ -831,6 +831,38 @@ bool nbcon_exit_unsafe(struct nbcon_writ
 EXPORT_SYMBOL_GPL(nbcon_exit_unsafe);
 
 /**
+ * nbcon_reacquire - Reacquire a console after losing ownership
+ * @wctxt:	The write context that was handed to the write function
+ *
+ * Since ownership can be lost at any time due to handover or takeover, a
+ * printing context _should_ be prepared to back out immediately and
+ * carefully. However, there are many scenarios where the context _must_
+ * reacquire ownership in order to finalize or revert hardware changes.
+ *
+ * This function allows a context to reacquire ownership using the same
+ * priority as its previous ownership.
+ *
+ * Note that for printing contexts, after a successful reacquire the
+ * context will have no output buffer because that has been lost. This
+ * function cannot be used to resume printing.
+ */
+void nbcon_reacquire(struct nbcon_write_context *wctxt)
+{
+	struct nbcon_context *ctxt = &ACCESS_PRIVATE(wctxt, ctxt);
+	struct console *con = ctxt->console;
+	struct nbcon_state cur;
+
+	while (!nbcon_context_try_acquire(ctxt))
+		cpu_relax();
+
+	wctxt->outbuf = NULL;
+	wctxt->len = 0;
+	nbcon_state_read(con, &cur);
+	wctxt->unsafe_takeover = cur.unsafe_takeover;
+}
+EXPORT_SYMBOL_GPL(nbcon_reacquire);
+
+/**
  * nbcon_emit_next_record - Emit a record in the acquired context
  * @wctxt:	The write context that will be handed to the write function
  * @use_atomic:	True if the write_atomic callback is to be used
