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Message-ID: <20260206165002.496724-6-pmladek@suse.com>
Date: Fri, 6 Feb 2026 17:49:59 +0100
From: Petr Mladek <pmladek@...e.com>
To: John Ogness <john.ogness@...utronix.de>
Cc: Sergey Senozhatsky <senozhatsky@...omium.org>,
Steven Rostedt <rostedt@...dmis.org>,
Marcos Paulo de Souza <mpdesouza@...e.com>,
Chris Down <chris@...isdown.name>,
linux-kernel@...r.kernel.org,
Petr Mladek <pmladek@...e.com>
Subject: [PATCH 5/8] printk: Try to register each console as Braille first
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. The logic relies on non-obvious shortcuts within both
try_enable_preferred_console() and register_console(), making the
initialization flow fragile and difficult to follow.
Refactor the logic by adding a parameter to try_enable_preferred_console()
to explicitly handle Braille vs. non-Braille cases. This removes the
need for implicit shortcuts in register_console(). 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.
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@...e.com>
---
kernel/printk/printk.c | 56 ++++++++++++++++++++++++++++++++++--------
1 file changed, 46 insertions(+), 10 deletions(-)
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 4b2865831a62..279b36ef90bd 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -365,6 +365,7 @@ 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);
@@ -2554,7 +2555,9 @@ static int update_preferred_console(int i, const char *name, const short idx,
braille_update_options(pc, brl_options);
- if (!brl_options)
+ if (brl_options)
+ want_braille_console = true;
+ else
preferred_dev_console = i;
/*
@@ -3900,8 +3903,9 @@ 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;
@@ -3918,6 +3922,12 @@ static int try_enable_preferred_console(struct console *newcon,
newcon->match(newcon, pc->name, pc->index, pc->options) != 0) {
/* default matching */
BUILD_BUG_ON(sizeof(pc->name) != sizeof(newcon->name));
+ /*
+ * Two entries might have the same pc->name when one was
+ * defined via "devname".
+ */
+ if (try_only_braille && !is_braille_console_preferred(pc))
+ continue;
if (strcmp(pc->name, newcon->name) != 0)
continue;
if (newcon->index >= 0 &&
@@ -3926,7 +3936,7 @@ 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);
@@ -3950,6 +3960,17 @@ 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)
{
@@ -4103,6 +4124,20 @@ void register_console(struct console *newcon)
goto unlock;
}
+ /*
+ * 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.
+ */
+ if (want_braille_console) {
+ err = try_enable_braille_console(newcon);
+ /* Return on success or when con->setup failed. */
+ if (err != -ENOENT)
+ goto unlock_free;
+ }
+
/*
* See if we want to enable this console driver by default.
*
@@ -4129,12 +4164,8 @@ void register_console(struct console *newcon)
if (err == -ENOENT)
err = try_enable_preferred_console(newcon, false);
- /* printk() messages are not printed to the Braille console. */
- if (err || newcon->flags & CON_BRL) {
- if (newcon->flags & CON_NBCON)
- nbcon_free(newcon);
- goto unlock;
- }
+ if (err)
+ goto unlock_free;
/*
* If we have a bootconsole, and are switching to a real console,
@@ -4227,6 +4258,11 @@ void register_console(struct console *newcon)
printk_kthreads_check_locked();
unlock:
console_list_unlock();
+ return;
+
+unlock_free:
+ nbcon_free(newcon);
+ goto unlock;
}
EXPORT_SYMBOL(register_console);
--
2.52.0
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