[PATCH v5 16/16] printk: Try enable preferred consoles only when there are any

[Petr Mladek <pmladek@suse.com>]

try_enable_preferred_console() used to be always called because it
had several hidden effects, namely:

- returned success when a console was pre-enabled using CON_ENABLED
  flag.

- enabled Braille consoles which were ignored by "preferred_dev_console"
  because they were not associated with /dev/console.

- returned success when a console was enabled by default because
  try_enable_default_console() did not return success.

The first two hidden effects were removed in previous patches. The already
fixed handling of pre-enabled consoles actually helps even the 3rd case.
try_enable_default_console() sets CON_ENABLED flag on success and
is later handled as pre-enabled.

Prevent any future hidden effects and call try_enable_preferred_console()
only when some console is preferred.

No behavior change.

Signed-off-by: Petr Mladek <pmladek@suse.com>
---
 kernel/printk/printk.c | 26 +++++++++++++++++---------
 1 file changed, 17 insertions(+), 9 deletions(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 1eb749da9e24..06c49042dd8f 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -4085,7 +4085,7 @@ static int try_enable_default_console(struct console *newcon)
 
 static int try_enable_console(struct console *newcon)
 {
-	int err;
+	int err = -ENOENT;
 
 	/*
 	 * First, try to enable the console driver as a Braille console.
@@ -4124,16 +4124,24 @@ static int try_enable_console(struct console *newcon)
 			if (err != -ENOENT)
 				return err;
 		}
+	} else {
+		/* See if this console matches one we selected on the command line */
+		err = try_enable_preferred_console(newcon, true);
+		if (err != -ENOENT)
+			return err;
+
+		/* If not, try to match against the platform default(s) */
+		err = try_enable_preferred_console(newcon, false);
+		if (err != -ENOENT)
+			return err;
 	}
 
-	/* See if this console matches one we selected on the command line */
-	err = try_enable_preferred_console(newcon, true);
-	if (err != -ENOENT)
-		return err;
-
-	/* If not, try to match against the platform default(s) */
-	err = try_enable_preferred_console(newcon, false);
-	if (err != -ENOENT)
+	/*
+	 * Make sure that pre-enabled consoles won't get registered when
+	 * something went wrong. This is just a paranoid check that all
+	 * branches above returned on error.
+	 */
+	if (WARN_ON(err != -ENOENT))
 		return err;
 
 	/*
-- 
2.55.0