console: prevent registered consoles from dumping old kernel message over again

For a platform with many consoles like:
 "console=tty1 console=ttyMFD2 console=ttyS0 earlyprintk=mrst"

Each time when the non "selected_console" (tty1 and ttyMFD2 here) get
registered, the existing kernel message will be printed out on registered
consoles again, the "mrst" early console will get some same message for 3
times, and "tty1" will get some for twice.

As suggested by Andrew Morton, every time a new console is registered, it
will be set as the "exclusive" console which will dump the already
existing kernel messages.

Signed-off-by: Feng Tang <feng.tang@intel.com>
Cc: Greg KH <gregkh@suse.de>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 18 insertions, 0 deletions

diff --git a/kernel/printk.c b/kernel/printk.c
index 2b591f252e55..a53607eea6d0 100644
--- a/kernel/printk.c
+++ b/kernel/printk.c
@@ -113,6 +113,11 @@ static unsigned con_start;	/* Index into log_buf: next char to be sent to consol
 static unsigned log_end;        /* Index into log_buf: most-recently-written-char + 1 */
 
 /*
+ * If exclusive_console is non-NULL then only this console is to be printed to.
+ */
+static struct console *exclusive_console;
+
+/*
  *      Array of consoles built from command line options (console=)
  */
 struct console_cmdline
@@ -476,6 +481,8 @@ static void __call_console_drivers(unsigned start, unsigned end)
         struct console *con;
 
         for_each_console(con) {
+                if (exclusive_console && con != exclusive_console)
+                        continue;
                 if ((con->flags & CON_ENABLED) && con->write &&
                                 (cpu_online(smp_processor_id()) ||
                                 (con->flags & CON_ANYTIME)))
@@ -1230,6 +1237,11 @@ void console_unlock(void)
                 local_irq_restore(flags);
         }
         console_locked = 0;
+
+        /* Release the exclusive_console once it is used */
+        if (unlikely(exclusive_console))
+                exclusive_console = NULL;
+
         up(&console_sem);
         spin_unlock_irqrestore(&logbuf_lock, flags);
         if (wake_klogd)
@@ -1464,6 +1476,12 @@ void register_console(struct console *newcon)
                 spin_lock_irqsave(&logbuf_lock, flags);
                 con_start = log_start;
                 spin_unlock_irqrestore(&logbuf_lock, flags);
+                /*
+                 * We're about to replay the log buffer.  Only do this to the
+                 * just-registered console to avoid excessive message spam to
+                 * the already-registered consoles.
+                 */
+                exclusive_console = newcon;
         }
         console_unlock();
         console_sysfs_notify();