printk: fix double printing with earlycon

If a console was specified by ACPI SPCR table _and_ command line
parameters like "console=ttyAMA0" _and_ "earlycon" were specified,
then log messages appear twice.

The root cause is that the code traverses the list of specified
consoles (the `console_cmdline` array) and stops at the first match.
But it may happen that the same console is referred by the elements
of this array twice:

	pl011,mmio,0x87e024000000,115200 -- from SPCR
	ttyAMA0 -- from command line

but in this case `preferred_console` points to the second entry and
the flag CON_CONSDEV is not set, so bootconsole is not deregistered.

To fix that, introduce an invariant "The last non-braille console
is always the preferred one" on the entries of the console_cmdline
array.  Then traverse it in reverse order to be sure that if
the console is preferred then it will be the first matching entry.
Introduce variable console_cmdline_cnt that keeps the number
of elements of the console_cmdline array (Petr Mladek).  It helps
to get rid of the loop that searches for the end of this array.

Link: http://lkml.kernel.org/r/20170405202006.18234-1-aleksey.makarov@linaro.org
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Peter Hurley <peter@hurleysoftware.com>
Cc: Jiri Slaby <jslaby@suse.com>
Cc: Robin Murphy <robin.murphy@arm.com>
Cc: "Nair, Jayachandran" <Jayachandran.Nair@cavium.com>
Cc: linux-serial@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Aleksey Makarov <aleksey.makarov@linaro.org>
Reported-by: Sudeep Holla <sudeep.holla@arm.com>
Acked-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>

kernel/printk/printk.c

@@ -266,6 +266,7 @@ static struct console *exclusive_console;
 #define MAX_CMDLINECONSOLES 8
 
 static struct console_cmdline console_cmdline[MAX_CMDLINECONSOLES];
+static int console_cmdline_cnt;
 
 static int preferred_console = -1;
 int console_set_on_cmdline;
@@ -1902,12 +1903,25 @@ static int __add_preferred_console(char *name, int idx, char *options,
 	 *	See if this tty is not yet registered, and
 	 *	if we have a slot free.
 	 */
-	for (i = 0, c = console_cmdline;
-	     i < MAX_CMDLINECONSOLES && c->name[0];
-	     i++, c++) {
+	for (i = 0, c = console_cmdline; i < console_cmdline_cnt; i++, c++) {
 		if (strcmp(c->name, name) == 0 && c->index == idx) {
-			if (!brl_options)
-				preferred_console = i;
+			if (brl_options)
+				return 0;
+
+			/*
+			 * Maintain an invariant that will help to find if
+			 * the matching console is preferred, see
+			 * register_console():
+			 *
+			 * The last non-braille console is always
+			 * the preferred one.
+			 */
+			if (i != console_cmdline_cnt - 1)
+				swap(console_cmdline[i],
+				     console_cmdline[console_cmdline_cnt - 1]);
+
+			preferred_console = console_cmdline_cnt - 1;
+
 			return 0;
 		}
 	}
@@ -1920,6 +1934,7 @@ static int __add_preferred_console(char *name, int idx, char *options,
 	braille_set_options(c, brl_options);
 
 	c->index = idx;
+	console_cmdline_cnt++;
 	return 0;
 }
 /*
@@ -2459,12 +2474,23 @@ void register_console(struct console *newcon)
 	}
 
 	/*
-	 *	See if this console matches one we selected on
-	 *	the command line.
+	 * See if this console matches one we selected on the command line.
+	 *
+	 * There may be several entries in the console_cmdline array matching
+	 * with the same console, one with newcon->match(), another by
+	 * name/index:
+	 *
+	 *	pl011,mmio,0x87e024000000,115200 -- added from SPCR
+	 *	ttyAMA0 -- added from command line
+	 *
+	 * Traverse the console_cmdline array in reverse order to be
+	 * sure that if this console is preferred then it will be the first
+	 * matching entry.  We use the invariant that is maintained in
+	 * __add_preferred_console().
 	 */
-	for (i = 0, c = console_cmdline;
-	     i < MAX_CMDLINECONSOLES && c->name[0];
-	     i++, c++) {
+	for (i = console_cmdline_cnt - 1; i >= 0; i--) {
+		c = console_cmdline + i;
+
 		if (!newcon->match ||
 		    newcon->match(newcon, c->name, c->index, c->options) != 0) {
 			/* default matching */