[PATCH V3 08/10] tty: serial: of-serial: Suppress warnings if OF earlycon is invoked twice

[Kevin Cernekee <cernekee@gmail.com>]

Specifying "earlycon earlycon" on the kernel command line yields this
warning:

    bootconsole [uart0] enabled
    ------------[ cut here ]------------
    WARNING: CPU: 0 PID: 0 at kernel/printk/printk.c:2391 register_console+0x244/0x3fc()
    console 'uart0' already registered
    CPU: 0 PID: 0 Comm: swapper Not tainted 3.18.0-rc1+ #2
    Stack : 00000000 00000004 80af0000 80af0000 00000000 00000000 00000000 00000000
              80ad4e12 00000036 00000000 00000000 00010000 805abe88 805606b4 805abae7
              00000000 00000000 80ad38d8 805abe88 8055f304 43d42d03 9988c6a1 804e2710
              805b0000 80032854 00000000 00000000 8056492c 80599c84 80599c84 805606b4
              00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000
              ...
    Call Trace:
    [<8001a22c>] show_stack+0x64/0x7c
    [<804e47d8>] dump_stack+0xc8/0xfc
    [<80032aa8>] warn_slowpath_common+0x7c/0xac
    [<80032b38>] warn_slowpath_fmt+0x2c/0x38
    [<80076524>] register_console+0x244/0x3fc
    [<805d8314>] of_setup_earlycon+0x74/0x98
    [<805daa40>] early_init_dt_scan_chosen_serial+0x104/0x134
    [<805c51a0>] do_early_param+0xc4/0x13c
    [<8004efa0>] parse_args+0x284/0x444
    [<805c56cc>] parse_early_options+0x34/0x40
    [<805c5714>] parse_early_param+0x3c/0x58
    [<805c87a4>] setup_arch+0xec/0x6e4
    [<805c57d4>] start_kernel+0x94/0x458

    ---[ end trace dc8fa200cb88537f ]---

In this case the duplicate "earlycon" was entered directly, but there are
other cases where this could happen inadvertently:

 - Some platforms allow user bootargs to be concatenated with builtin
   bootargs, e.g. CONFIG_CMDLINE_EXTEND.

 - Other platforms may want to hardwire earlycon to ON, so it isn't
   nice if a user manually specifying "earlycon" on the command line sees
   a big scary warning.

So, we will treat "earlycon" as a flag, and if happens to be requested
multiple times the kernel will not print any warnings.

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
---
 drivers/of/fdt.c | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/drivers/of/fdt.c b/drivers/of/fdt.c
index d1ffca8..20193cc 100644
--- a/drivers/of/fdt.c
+++ b/drivers/of/fdt.c
@@ -755,6 +755,11 @@ int __init early_init_dt_scan_chosen_serial(void)
 	int l;
 	const struct of_device_id *match = __earlycon_of_table;
 	const void *fdt = initial_boot_params;
+	static int done;
+
+	if (done)
+		return -EBUSY;
+	done = 1;
 
 	offset = fdt_path_offset(fdt, "/chosen");
 	if (offset < 0)
@@ -792,10 +797,9 @@ int __init early_init_dt_scan_chosen_serial(void)
 
 static int __init setup_of_earlycon(char *buf)
 {
-	if (buf)
-		return 0;
-
-	return early_init_dt_scan_chosen_serial();
+	if (!buf)
+		early_init_dt_scan_chosen_serial();
+	return 0;
 }
 early_param("earlycon", setup_of_earlycon);
 #endif
-- 
2.1.1