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Date:   Fri, 15 Nov 2019 13:33:56 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
To:     Petr Mladek <pmladek@suse.com>
Cc:     Jonathan Richardson <jonathan.richardson@broadcom.com>,
        gregkh@linuxfoundation.org, jslaby@suse.com,
        sergey.senozhatsky@gmail.com, linux-serial@vger.kernel.org,
        linux-kernel@vger.kernel.org,
        Scott Branden <scott.branden@broadcom.com>,
        Ray Jui <ray.jui@broadcom.com>,
        Srinath Mannam <srinath.mannam@broadcom.com>
Subject: Re: console output duplicated when registering additional consoles
Message-ID: <20191115043356.GA220831@google.com>
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Gosh, that part of printk is really complex.

On (19/11/14 10:57), Petr Mladek wrote:
> For a proper solution we would need to match boot and real
> consoles that write messages into the physical device.
> But I am afraid that there is no support for this.

Wouldn't those have same tty driver?

---

 kernel/printk/printk.c | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index f1b08015d3fa..a84cb20acf42 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -2690,6 +2690,19 @@ static int __init keep_bootcon_setup(char *str)
 
 early_param("keep_bootcon", keep_bootcon_setup);
 
+static bool known_console_driver(struct console *newcon)
+{
+	struct console *con;
+
+	for_each_console(con) {
+		if (!(con->flags & CON_ENABLED))
+			continue;
+		if (con->device && con->device == newcon->device)
+			return true;
+	}
+	return false;
+}
+
 /*
  * The console driver calls this routine during kernel initialization
  * to register the console printing procedure with printk() and to
@@ -2828,6 +2841,9 @@ void register_console(struct console *newcon)
 	if (newcon->flags & CON_EXTENDED)
 		nr_ext_console_drivers++;
 
+	if (known_console_driver(newcon))
+		newcon->flags &= ~CON_PRINTBUFFER;
+
 	if (newcon->flags & CON_PRINTBUFFER) {
 		/*
 		 * console_unlock(); will print out the buffered messages