[PATCH v2 6/9] printk: Try to register each console as Braille first

[Petr Mladek]

Braille consoles are unique: they can only be enabled via the command
line and use the preferred console framework for initialization, yet
they are never added to the console_list or associated with
/dev/console. Because of this, the preferred_console variable remains
unset when only a Braille console is requested.

Currently, try_enable_preferred_console() must be called even when
"preferred_dev_console" variable is not set, just to catch these "hidden"
Braille requests.

Refactor the logic by adding a parameter to try_enable_preferred_console()
to explicitly handle Braille vs. non-Braille cases. It will eventually
allow to skip try_enable_preferred_console() when there are no preferred
consoles. This improves code robustness by ensuring the console
setup is explicit and only performed once.

Note that _braille_register_console() is skipped when the given console
driver has a match() callback and it succeeded. It means that it took
over an early console and can't be used as a Braille console.

As a result try_enable_braille_console() might return success even
when it did not enabled the console in the Braille mode. It is
the reason why register_console() has to explicitly check whether
CON_BRL flag was really set and could skip the further steps.

The refactoring even fixes two subtle bugs:

1. When only the Braille console is defined on the command line,
   the original code might attempt to enable the same console twice—once
   as a default and once as a Braille console. This results in calling
   the setup() callback twice and incorrectly setting the CON_CONSDEV flag.

2. When the same console is defined on the command line using devname
   and then as Braille console, for example:

      console=00:00:0.0,115200 console=brl,ttyS0,115200

   It would have two separate entries in preferred_consoles[] array.

   The 2nd (Braille) entry would be used when univ8250_console_init()
   tries to register the generic "ttyS" console driver. Note that
   the 1st entry still does not have defined the "name" entry at
   this stage.

   The 1st non-Braille entry would be used later when serial8250_init()
   registers all found devices, assigns "tty0" for the given "00:00:0.0"
   devname and tries to register the same struct console once again.

   The original code would call newcon->setup() twice in this scenario.
   It won't add the console into console_list only because the later
   check in register_console() would detect CON_BRL flag set from
   the 1st registration and return early.

Signed-off-by: Petr Mladek <pmladek@xxxxxxxx>
---
 kernel/printk/printk.c | 56 +++++++++++++++++++++++++++++++++++++-----
 1 file changed, 50 insertions(+), 6 deletions(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 2543c810efcb..e34955189d16 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -367,6 +367,7 @@ static int console_locked;
 static struct preferred_console preferred_consoles[MAX_PREFERRED_CONSOLES];
 
 static int preferred_dev_console = -1;
+static bool want_braille_console;
 int console_set_on_cmdline;
 EXPORT_SYMBOL(console_set_on_cmdline);
 
@@ -2644,7 +2645,9 @@ static int update_preferred_console(unsigned int i,
 
 	braille_update_options(pc, brl_options);
 
-	if (!brl_options)
+	if (brl_options)
+		want_braille_console = true;
+	else
 		preferred_dev_console = i;
 
 	/*
@@ -3975,8 +3978,9 @@ static int console_call_setup(struct console *newcon, char *options)
  * Care need to be taken with consoles that are statically
  * enabled such as netconsole
  */
-static int try_enable_preferred_console(struct console *newcon,
-					bool user_specified)
+static int __try_enable_preferred_console(struct console *newcon,
+					  bool user_specified,
+					  bool try_only_braille)
 {
 	struct preferred_console *pc;
 	int i, err;
@@ -3987,8 +3991,19 @@ static int try_enable_preferred_console(struct console *newcon,
 		/* Console not yet initialized? */
 		if (!pc->name[0])
 			continue;
-		if (pc->user_specified != user_specified)
-			continue;
+
+		/*
+		 * @try_only_braille and @user_specifified define which
+		 * preferred console entries are handled in this round.
+		 */
+		if (try_only_braille) {
+			if (!is_braille_console_preferred(pc))
+				continue;
+		} else {
+			if (pc->user_specified != user_specified)
+				continue;
+		}
+
 		if (!newcon->match ||
 		    newcon->match(newcon, pc->name, pc->index, pc->options) != 0) {
 			/* default matching */
@@ -4001,7 +4016,7 @@ static int try_enable_preferred_console(struct console *newcon,
 			if (newcon->index < 0)
 				newcon->index = pc->index;
 
-			if (is_braille_console_preferred(pc))
+			if (try_only_braille)
 				return _braille_register_console(newcon, pc);
 
 			err = console_call_setup(newcon, pc->options);
@@ -4025,6 +4040,17 @@ static int try_enable_preferred_console(struct console *newcon,
 	return -ENOENT;
 }
 
+static int try_enable_preferred_console(struct console *newcon,
+					bool user_specified)
+{
+	return __try_enable_preferred_console(newcon, user_specified, false);
+}
+
+static int try_enable_braille_console(struct console *newcon)
+{
+	return __try_enable_preferred_console(newcon, true, true);
+}
+
 /* Try to enable the console unconditionally */
 static void try_enable_default_console(struct console *newcon)
 {
@@ -4047,6 +4073,24 @@ static int try_enable_console(struct console *newcon)
 {
 	int err;
 
+	/*
+	 * First, try to enable the console driver as a Braille console.
+	 * It would have metadata in the preferred_consoles[] array.
+	 * But it won't be counted as @preferred_console because
+	 * it does not get printk() messages and is not associated
+	 * with /dev/console.
+	 *
+	 * Note that it might succeed also when the driver has a match()
+	 * callback and it took over a boot console. In this case,
+	 * the driver will continue working as a classic non-Braille
+	 * console.
+	 */
+	if (want_braille_console) {
+		err = try_enable_braille_console(newcon);
+		if (err != -ENOENT)
+			return err;
+	}
+
 	/*
 	 * See if we want to enable this console driver by default.
 	 *
-- 
2.53.0