[PATCH 1/2] printk: nbcon: Introduce Braille helpers

[Petr Mladek]

These helpers will be used when calling console->write_atomic
in the Braille console driver.

The Braille console is not registered as a standard printk console.
Instead, it is integrated with the virtual terminal (VT) and writes
data using the legacy con->write() callback of the associated serial
console driver.

When the underlying console driver is converted to the NBCON API, the
Braille console needs to use the con->write_atomic() callback instead.
This callback must be synchronized by acquiring the nbcon console context.

Fixes: d3539347022a ("serial: 8250: Switch to nbcon console, take 2")
Signed-off-by: Petr Mladek <pmladek@xxxxxxxx>
---
 include/linux/console.h |  8 ++++++
 kernel/printk/nbcon.c   | 59 +++++++++++++++++++++++++++++++++++++++++
 2 files changed, 67 insertions(+)

diff --git a/include/linux/console.h b/include/linux/console.h
index 502d1abe3f50..d780f6de303a 100644
--- a/include/linux/console.h
+++ b/include/linux/console.h
@@ -615,6 +615,10 @@ extern bool nbcon_allow_unsafe_takeover(void);
 extern bool nbcon_kdb_try_acquire(struct console *con,
 				  struct nbcon_write_context *wctxt);
 extern void nbcon_kdb_release(struct nbcon_write_context *wctxt);
+extern bool nbcon_is_braille(struct nbcon_write_context *wctxt);
+extern bool nbcon_braille_try_acquire(struct console *con,
+				      struct nbcon_write_context *wctxt);
+extern void nbcon_braille_release(struct nbcon_write_context *wctxt);
 
 /*
  * Check if the given console is currently capable and allowed to print
@@ -678,8 +682,12 @@ static inline void nbcon_reacquire_nobuf(struct nbcon_write_context *wctxt) { }
 static inline bool nbcon_kdb_try_acquire(struct console *con,
 					 struct nbcon_write_context *wctxt) { return false; }
 static inline void nbcon_kdb_release(struct nbcon_write_context *wctxt) { }
+static inline bool nbcon_is_braille(struct nbcon_write_context *wctxt) { return false; }
 static inline bool console_is_usable(struct console *con, short flags,
 				     bool use_atomic) { return false; }
+static inline bool nbcon_braille_try_acquire(struct console *con,
+					     struct nbcon_write_context *wctxt) { return false; }
+static inline void nbcon_braille_release(struct nbcon_write_context *wctxt) { }
 #endif
 
 extern int console_set_on_cmdline;
diff --git a/kernel/printk/nbcon.c b/kernel/printk/nbcon.c
index d17704fe93ae..a5e053ffe4da 100644
--- a/kernel/printk/nbcon.c
+++ b/kernel/printk/nbcon.c
@@ -2002,3 +2002,62 @@ void nbcon_kdb_release(struct nbcon_write_context *wctxt)
 	 */
 	__nbcon_atomic_flush_pending_con(ctxt->console, prb_next_reserve_seq(prb));
 }
+
+/**
+ * nbcon_is_braille - Checks whether the nbcon write context is using Braille console
+ *
+ * @wctxt:	checked nbcon write context
+ *
+ * Return: True when the write context is associated with a Braille console.
+ *	   Othrewise, return false.
+ *
+ * Context: Can be called in any context but only when Braille console is
+ *	registered and the struct console could not disappear.
+ */
+bool nbcon_is_braille(struct nbcon_write_context *wctxt)
+{
+	struct nbcon_context *ctxt = &ACCESS_PRIVATE(wctxt, ctxt);
+	struct console *con = ctxt->console;
+
+	return con && con->flags & CON_BRL;
+}
+
+/**
+ * nbcon_braille_try_acquire - Try to acquire nbcon console for braille_write()
+ *
+ * @con:	The nbcon console to acquire
+ * @wctxt:	The nbcon write context to be used on success
+ *
+ * Context:	braille_write() for emitting a single buffer on Braille console.
+ *
+ * Return:	True if the console was acquired. False otherwise.
+ *
+ * Braille console is not registered as a proper printk consoles. Instead,
+ * it is integrated with the graphical virtual terminal.
+ *
+ * This function acquires the nbcon console using priority NBCON_PRIO_EMERGENCY.
+ */
+bool nbcon_braille_try_acquire(struct console *con,
+			   struct nbcon_write_context *wctxt)
+{
+	struct nbcon_context *ctxt = &ACCESS_PRIVATE(wctxt, ctxt);
+
+	memset(ctxt, 0, sizeof(*ctxt));
+	ctxt->console = con;
+	ctxt->prio    = NBCON_PRIO_EMERGENCY;
+
+	return nbcon_context_try_acquire(ctxt, false);
+}
+
+/**
+ * nbcon_braille_release - Release the nbcon console
+ *
+ * @wctxt:	The nbcon write context initialized by a successful
+ *		nbcon_braille_try_acquire()
+ */
+void nbcon_braille_release(struct nbcon_write_context *wctxt)
+{
+	struct nbcon_context *ctxt = &ACCESS_PRIVATE(wctxt, ctxt);
+
+	nbcon_context_release(ctxt);
+}
-- 
2.55.0