[RFC PATCH 1/3] printk: Add function to set console to preferred console's driver

[Alper Nebi Yasak <alpernebiyasak@gmail.com>]

Currently, add_preferred_console sets a preferred console, but doesn't
actually change /dev/console to match it. That part is handled within
register_device, where a newly registered console driver will be set as
/dev/console if it matches the preferred console.

However, if the relevant driver is already registered, the only way to
set it as /dev/console is by un-registering and re-registering it. An
example is the xenfb_make_preferred_console() function:

	console_lock();
	for_each_console(c) {
		if (!strcmp(c->name, "tty") && c->index == 0)
			break;
	}
	console_unlock();
	if (c) {
		unregister_console(c);
		c->flags |= CON_CONSDEV;
		c->flags &= ~CON_PRINTBUFFER; /* don't print again */
		register_console(c);
	}

The code above was introduced in commit 9e124fe16ff2. In short, it's aim
is to set VT as the preferred console only after a working framebuffer
is registered and thus VT is not the dummy device.

This patch introduces an update_console_to_preferred function that
handles the necessary /dev/console change. With this change, the example
above can be replaced with:

	console_lock();
	add_preferred_console("tty", 0, NULL);
	update_console_to_preferred();
	console_unlock();

More importantly, these two calls can be moved to vt.c in order to bump
its priority when a non-dummy backend for it is introduced, solving that
problem in general.

Signed-off-by: Alper Nebi Yasak <alpernebiyasak@gmail.com>
---
 include/linux/console.h |  1 +
 kernel/printk/printk.c  | 56 +++++++++++++++++++++++++++++++++++++++++
 2 files changed, 57 insertions(+)

diff --git a/include/linux/console.h b/include/linux/console.h
index 75dd20650fb..4b3fa34be24 100644
--- a/include/linux/console.h
+++ b/include/linux/console.h
@@ -172,6 +172,7 @@ enum con_flush_mode {
 };
 
 extern int add_preferred_console(char *name, int idx, char *options);
+extern int update_console_to_preferred(void);
 extern void register_console(struct console *);
 extern int unregister_console(struct console *);
 extern struct console *console_drivers;
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index ad460623454..6b16c973587 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -2212,12 +2212,68 @@ __setup("console=", console_setup);
  * be used by arch-specific code either to override the user or more
  * commonly to provide a default console (ie from PROM variables) when
  * the user has not supplied one.
+ *
+ * Preferences set by this function don't take effect until the next
+ * time a matching driver for the preferred console is registered. If a
+ * matching driver was already registered, @update_console_to_preferred
+ * function can be used to set that as the preferred console driver.
  */
 int add_preferred_console(char *name, int idx, char *options)
 {
 	return __add_preferred_console(name, idx, options, NULL, false);
 }
 
+/**
+ * update_console_to_preferred - set console to the preferred console's driver.
+ *
+ * Updates console_drivers and CON_CONSDEV flags so that an already
+ * registered and enabled console driver matching the preferred console
+ * is used as /dev/console.
+ *
+ * Must be called within console_lock();.
+ */
+int update_console_to_preferred(void)
+{
+	struct console_cmdline *c = NULL;
+	struct console *con = NULL;
+	struct console *tmp = NULL;
+
+	if (preferred_console >= 0)
+		c = &console_cmdline[preferred_console];
+
+	if (!c || !c->name[0])
+		return 0;
+
+	for_each_console(con) {
+		if (!con->next || !(con->next->flags & CON_ENABLED))
+			continue;
+		if (strcmp(c->name, con->next->name) != 0)
+			continue;
+		if (con->next->index >= 0 &&
+		    con->next->index != c->index)
+			continue;
+		break;
+	}
+
+	if (!con)
+		return -ENODEV;
+
+	pr_info("switching to console [%s%d]\n",
+		con->next->name, con->next->index);
+
+	tmp = con->next;
+	con->next = con->next->next;
+	tmp->next = console_drivers;
+	console_drivers = tmp;
+
+	if (console_drivers->next)
+		console_drivers->next->flags &= ~CON_CONSDEV;
+	console_drivers->flags |= CON_CONSDEV;
+	has_preferred_console = true;
+
+	return 0;
+}
+
 bool console_suspend_enabled = true;
 EXPORT_SYMBOL(console_suspend_enabled);
 
-- 
2.26.0.rc2