From: John Ogness <john.ogness@linutronix.de>
Date: Tue, 12 Feb 2019 15:29:51 +0100
Subject: [PATCH 13/25] printk: track seq per console
Origin: https://www.kernel.org/pub/linux/kernel/projects/rt/5.4/older/patches-5.4.13-rt7.tar.xz

Allow each console to track which seq record was last printed. This
simplifies identifying dropped records.

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  |   30 +++++++++++++++++++++++++++---
 2 files changed, 28 insertions(+), 3 deletions(-)

--- a/include/linux/console.h
+++ b/include/linux/console.h
@@ -153,6 +153,7 @@ struct console {
 	short	flags;
 	short	index;
 	int	cflag;
+	unsigned long printk_seq;
 	void	*data;
 	struct	 console *next;
 };
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -1507,6 +1507,16 @@ SYSCALL_DEFINE3(syslog, int, type, char
 	return do_syslog(type, buf, len, SYSLOG_FROM_READER);
 }
 
+static void print_console_dropped(struct console *con, u64 count)
+{
+	char text[64];
+	int len;
+
+	len = sprintf(text, "** %llu printk message%s dropped **\n",
+		      count, count > 1 ? "s" : "");
+	con->write(con, text, len);
+}
+
 static void format_text(struct printk_log *msg, u64 seq,
 			char *ext_text, size_t *ext_len,
 			char *text, size_t *len, bool time)
@@ -1540,7 +1550,7 @@ static void format_text(struct printk_lo
  * log_buf[start] to log_buf[end - 1].
  * The console_lock must be held.
  */
-static void call_console_drivers(const char *ext_text, size_t ext_len,
+static void call_console_drivers(u64 seq, const char *ext_text, size_t ext_len,
 				 const char *text, size_t len)
 {
 	struct console *con;
@@ -1558,6 +1568,19 @@ static void call_console_drivers(const c
 		if (!cpu_online(raw_smp_processor_id()) &&
 		    !(con->flags & CON_ANYTIME))
 			continue;
+		if (con->printk_seq >= seq)
+			continue;
+
+		con->printk_seq++;
+		if (con->printk_seq < seq) {
+			print_console_dropped(con, seq - con->printk_seq);
+			con->printk_seq = seq;
+		}
+
+		/* for supressed messages, only seq is updated */
+		if (len == 0 && ext_len == 0)
+			continue;
+
 		if (con->flags & CON_EXTENDED)
 			con->write(con, ext_text, ext_len);
 		else
@@ -1797,7 +1820,7 @@ static ssize_t msg_print_ext_header(char
 static ssize_t msg_print_ext_body(char *buf, size_t size,
 				  char *dict, size_t dict_len,
 				  char *text, size_t text_len) { return 0; }
-static void call_console_drivers(const char *ext_text, size_t ext_len,
+static void call_console_drivers(u64 seq, const char *ext_text, size_t ext_len,
 				 const char *text, size_t len) {}
 static size_t msg_print_text(const struct printk_log *msg, bool syslog,
 			     bool time, char *buf, size_t size) { return 0; }
@@ -2550,8 +2573,9 @@ static int printk_kthread_func(void *dat
 			    &len, printk_time);
 
 		console_lock();
+		call_console_drivers(master_seq, ext_text,
+				     ext_len, text, len);
 		if (len > 0 || ext_len > 0) {
-			call_console_drivers(ext_text, ext_len, text, len);
 			boot_delay_msec(msg->level);
 			printk_delay();
 		}