[PATCH v2] earlycon: prevent multiple register_console()

[Michael Walle <michael@walle.cc>]

If the earlycon parameter is given twice, the kernel will spit out a
WARN() in register_console() because it was already registered. The
non-dt variant setup_earlycon() already handles that gracefully. The dt
variant of_setup_earlycon() doesn't. Add the check there and add the
-EALREADY handling in early_init_dt_scan_chosen_stdout().

FWIW, this doesn't happen if CONFIG_ACPI_SPCR_TABLE is set. In that case
the registration is delayed until after earlycon parameter(s) are
parsed.

Signed-off-by: Michael Walle <michael@walle.cc>
---
changes since v1:
 - add missing EALREADY handling in of_setup_earlycon()
 - return 0 early as suggested by Rob

For the curious, here is the backtrace:

[    0.000000] ------------[ cut here ]------------
[    0.000000] WARNING: CPU: 0 PID: 0 at kernel/printk/printk.c:3328 register_console+0x2b4/0x364
[    0.000000] console 'atmel_serial0' already registered
[    0.000000] Modules linked in:
[    0.000000] CPU: 0 PID: 0 Comm: swapper Not tainted 5.18.0-next-20220601+ #652
[    0.000000] Hardware name: Generic DT based system
[    0.000000] Backtrace:
[    0.000000]  dump_backtrace from show_stack+0x18/0x1c
[    0.000000]  show_stack from dump_stack_lvl+0x48/0x54
[    0.000000]  dump_stack_lvl from dump_stack+0x18/0x1c
[    0.000000]  dump_stack from __warn+0xd0/0x148
[    0.000000]  __warn from warn_slowpath_fmt+0x9c/0xc4
[    0.000000]  warn_slowpath_fmt from register_console+0x2b4/0x364
[    0.000000]  register_console from of_setup_earlycon+0x29c/0x2ac
[    0.000000]  of_setup_earlycon from early_init_dt_scan_chosen_stdout+0x154/0x18c
[    0.000000]  early_init_dt_scan_chosen_stdout from param_setup_earlycon+0x40/0x48
[    0.000000]  param_setup_earlycon from do_early_param+0x88/0xc4
[    0.000000]  do_early_param from parse_args+0x1a4/0x404
[    0.000000]  parse_args from parse_early_options+0x40/0x48
[    0.000000]  parse_early_options from parse_early_param+0x38/0x48
[    0.000000]  parse_early_param from setup_arch+0x114/0x7a4
[    0.000000]  setup_arch from start_kernel+0x74/0x6dc
[    0.000000]  start_kernel from 0x0
[    0.000000] ---[ end trace 0000000000000000 ]---

 drivers/of/fdt.c              | 4 +++-
 drivers/tty/serial/earlycon.c | 3 +++
 2 files changed, 6 insertions(+), 1 deletion(-)

diff --git a/drivers/of/fdt.c b/drivers/of/fdt.c
index a8f5b6532165..043b12be22d6 100644
--- a/drivers/of/fdt.c
+++ b/drivers/of/fdt.c
@@ -1025,6 +1025,7 @@ int __init early_init_dt_scan_chosen_stdout(void)
 	int l;
 	const struct earlycon_id *match;
 	const void *fdt = initial_boot_params;
+	int ret;
 
 	offset = fdt_path_offset(fdt, "/chosen");
 	if (offset < 0)
@@ -1057,7 +1058,8 @@ int __init early_init_dt_scan_chosen_stdout(void)
 		if (fdt_node_check_compatible(fdt, offset, match->compatible))
 			continue;
 
-		if (of_setup_earlycon(match, offset, options) == 0)
+		ret = of_setup_earlycon(match, offset, options);
+		if (!ret || ret == -EALREADY)
 			return 0;
 	}
 	return -ENODEV;
diff --git a/drivers/tty/serial/earlycon.c b/drivers/tty/serial/earlycon.c
index 57c70851f22a..88d08ba1ca83 100644
--- a/drivers/tty/serial/earlycon.c
+++ b/drivers/tty/serial/earlycon.c
@@ -253,6 +253,9 @@ int __init of_setup_earlycon(const struct earlycon_id *match,
 	bool big_endian;
 	u64 addr;
 
+	if (early_con.flags & CON_ENABLED)
+		return -EALREADY;
+
 	spin_lock_init(&port->lock);
 	port->iotype = UPIO_MEM;
 	addr = of_flat_dt_translate_address(node);
-- 
2.30.2