[PATCH v6 13/16] 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.

Behavior change:

1. The Braille console driver is newly always preferred even when different
   serial ports are preferred. For example, let's have the following
   command lines:

     a) console=ttyS1,115200     console=brl,ttyS0,115200
     b) console=00:00:0.0,115200 console=brl,ttyS0,115200
     c) console=brl,ttyS0,115200 console=ttyS1,115200

   They all create two entries in preferred_consoles[] because:

     a) and c) differ in @index (0 vs. 1).
     b) defines either @name or @devname.

   But the original code registers:

     a) ttyS1 as normal console because it is the 1st entry in
	preferred_consoles[].

     b) ttyS0 are Braille console because the generic "ttyS" driver
	is registered early in univ8250_console_init(). The @devname
	entry is resolved too late when serial8250_init()
	registers all found devices, it assigns "tty0" for
	the given "00:00:0.0" but it is already in use.

      c) ttyS0 as Braille console because it is the 1st entry in
	 preferred_consoles[].

    The new code would register ttyS0 as the Braille console in
    all three variants because try_enable_braille_console() is
    called 1st.

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

The 1st change makes the behavior more consistent and understandable.
The 2nd change is an obvious bug fix.

Signed-off-by: Petr Mladek <pmladek@xxxxxxxx>
Acked-by: Chris Down <chris@xxxxxxxxxxxxxx>
---
 kernel/printk/printk.c | 58 +++++++++++++++++++++++++++++++++++++-----
 1 file changed, 52 insertions(+), 6 deletions(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 947574ee29a3..2bcf82b2c4e3 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -366,6 +366,7 @@ static int console_locked;
 static struct preferred_console preferred_consoles[MAX_PREFERRED_CONSOLES];
 
 static int preferred_dev_console = -1;
+static bool want_braille_console;
 /* Set when there is at least one console preferred via the command line. */
 int console_set_on_cmdline;
 EXPORT_SYMBOL(console_set_on_cmdline);
@@ -2648,7 +2649,9 @@ static int add_or_update_preferred_console(unsigned int i,
 		braille_set_options(pc, brl_options);
 	}
 
-	if (!brl_options)
+	if (brl_options)
+		want_braille_console = true;
+	else
 		preferred_dev_console = i;
 
 	if (user_specified) {
@@ -3984,8 +3987,9 @@ int console_call_setup(struct console *newcon, char *options)
  * by either the command line or add_preferred_console() and
  * setup/enable it.
  */
-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;
@@ -3996,8 +4000,21 @@ 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_specified define which
+		 * preferred console entries are handled in this round.
+		 */
+		if (try_only_braille) {
+			if (!is_braille_console_preferred(pc))
+				continue;
+		} else {
+			if (is_braille_console_preferred(pc))
+				continue;
+			if (pc->user_specified != user_specified)
+				continue;
+		}
+
 		if (!newcon->match ||
 		    newcon->match(newcon, pc->name, pc->index, pc->options) != 0) {
 			/* default matching */
@@ -4010,7 +4027,7 @@ 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);
@@ -4026,6 +4043,17 @@ 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)
 {
@@ -4048,6 +4076,24 @@ 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_dev_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.
 	 *
-- 
2.56.0