From: John Ogness <john.ogness@linutronix.de>
Date: Tue, 12 Feb 2019 15:29:53 +0100
Subject: [PATCH 15/25] printk: print history for new consoles
Origin: https://www.kernel.org/pub/linux/kernel/projects/rt/5.2/older/patches-5.2.10-rt5.tar.xz

When new consoles register, they currently print how many messages
they have missed. However, many (or all) of those messages may still
be in the ring buffer. Add functionality to print as much of the
history as available. This is a clean replacement of the old
exclusive console hack.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
 include/linux/console.h |    1 
 kernel/printk/printk.c  |   75 ++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 76 insertions(+)

--- a/include/linux/console.h
+++ b/include/linux/console.h
@@ -154,6 +154,7 @@ struct console {
 	short	index;
 	int	cflag;
 	unsigned long printk_seq;
+	int	wrote_history;
 	void	*data;
 	struct	 console *next;
 };
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -1550,6 +1550,77 @@ static void format_text(struct printk_lo
 	}
 }
 
+static void printk_write_history(struct console *con, u64 master_seq)
+{
+	struct prb_iterator iter;
+	bool time = printk_time;
+	static char *ext_text;
+	static char *text;
+	static char *buf;
+	u64 seq;
+
+	ext_text = kmalloc(CONSOLE_EXT_LOG_MAX, GFP_KERNEL);
+	text = kmalloc(PRINTK_SPRINT_MAX, GFP_KERNEL);
+	buf = kmalloc(PRINTK_RECORD_MAX, GFP_KERNEL);
+	if (!ext_text || !text || !buf)
+		return;
+
+	if (!(con->flags & CON_ENABLED))
+		goto out;
+
+	if (!con->write)
+		goto out;
+
+	if (!cpu_online(raw_smp_processor_id()) &&
+	    !(con->flags & CON_ANYTIME))
+		goto out;
+
+	prb_iter_init(&iter, &printk_rb, NULL);
+
+	for (;;) {
+		struct printk_log *msg;
+		size_t ext_len;
+		size_t len;
+		int ret;
+
+		ret = prb_iter_next(&iter, buf, PRINTK_RECORD_MAX, &seq);
+		if (ret == 0) {
+			break;
+		} else if (ret < 0) {
+			prb_iter_init(&iter, &printk_rb, NULL);
+			continue;
+		}
+
+		if (seq > master_seq)
+			break;
+
+		con->printk_seq++;
+		if (con->printk_seq < seq) {
+			print_console_dropped(con, seq - con->printk_seq);
+			con->printk_seq = seq;
+		}
+
+		msg = (struct printk_log *)buf;
+		format_text(msg, master_seq, ext_text, &ext_len, text,
+			    &len, time);
+
+		if (len == 0 && ext_len == 0)
+			continue;
+
+		if (con->flags & CON_EXTENDED)
+			con->write(con, ext_text, ext_len);
+		else
+			con->write(con, text, len);
+
+		printk_delay(msg->level);
+	}
+out:
+	con->wrote_history = 1;
+	kfree(ext_text);
+	kfree(text);
+	kfree(buf);
+}
+
 /*
  * Call the console drivers, asking them to write out
  * log_buf[start] to log_buf[end - 1].
@@ -1568,6 +1639,10 @@ static void call_console_drivers(u64 seq
 	for_each_console(con) {
 		if (!(con->flags & CON_ENABLED))
 			continue;
+		if (!con->wrote_history) {
+			printk_write_history(con, seq);
+			continue;
+		}
 		if (!con->write)
 			continue;
 		if (!cpu_online(raw_smp_processor_id()) &&