[PATCH v2] serial: 8250: Fix PM usage_count for console handover

["Ilpo Järvinen" <ilpo.jarvinen@linux.intel.com>]

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When console is enabled, univ8250_console_setup() calls
serial8250_console_setup() before .dev is set to uart_port. Therefore,
it will not call pm_runtime_get_sync(). Later, when the actual driver
is going to take over univ8250_console_exit() is called. As .dev is
already set, serial8250_console_exit() makes pm_runtime_put_sync() call
with usage count being zero triggering PM usage count warning
(extra debug for univ8250_console_setup(), univ8250_console_exit(), and
serial8250_register_ports()):

[    0.068987] univ8250_console_setup ttyS0 nodev
[    0.499670] printk: console [ttyS0] enabled
[    0.717955] printk: console [ttyS0] printing thread started
[    1.960163] serial8250_register_ports assigned dev for ttyS0
[    1.976830] printk: console [ttyS0] disabled
[    1.976888] printk: console [ttyS0] printing thread stopped
[    1.977073] univ8250_console_exit ttyS0 usage:0
[    1.977075] serial8250 serial8250: Runtime PM usage count underflow!
[    1.977429] dw-apb-uart.6: ttyS0 at MMIO 0x4010006000 (irq = 33, base_baud = 115200) is a 16550A
[    1.977812] univ8250_console_setup ttyS0 usage:2
[    1.978167] printk: console [ttyS0] printing thread started
[    1.978203] printk: console [ttyS0] enabled

To fix the issue, call pm_runtime_get_sync() in
serial8250_register_ports() as soon as .dev is set for an uart_port
if it has console enabled.

This problem became apparent only recently because 82586a721595 ("PM:
runtime: Avoid device usage count underflows") added the warning
printout. I confirmed this problem also occurs with v5.18 (w/o the
warning printout, obviously).

Fixes: bedb404e91bb ("serial: 8250_port: Don't use power management for kernel console")
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Tony Lindgren <tony@atomide.com>
Tested-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>

---
v2:
- Remove reference to reverted printk kthreads from the changelog
- Collect rev/tested-bys

 drivers/tty/serial/8250/8250_core.c | 4 ++++
 drivers/tty/serial/serial_core.c    | 5 -----
 include/linux/serial_core.h         | 5 +++++
 3 files changed, 9 insertions(+), 5 deletions(-)

diff --git a/drivers/tty/serial/8250/8250_core.c b/drivers/tty/serial/8250/8250_core.c
index 57e86133af4f..2e83e7367441 100644
--- a/drivers/tty/serial/8250/8250_core.c
+++ b/drivers/tty/serial/8250/8250_core.c
@@ -23,6 +23,7 @@
 #include <linux/sysrq.h>
 #include <linux/delay.h>
 #include <linux/platform_device.h>
+#include <linux/pm_runtime.h>
 #include <linux/tty.h>
 #include <linux/ratelimit.h>
 #include <linux/tty_flip.h>
@@ -558,6 +559,9 @@ serial8250_register_ports(struct uart_driver *drv, struct device *dev)
 
 		up->port.dev = dev;
 
+		if (uart_console_enabled(&up->port))
+			pm_runtime_get_sync(up->port.dev);
+
 		serial8250_apply_quirks(up);
 		uart_add_one_port(drv, &up->port);
 	}
diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
index 85ef7ef00b82..3161445504bc 100644
--- a/drivers/tty/serial/serial_core.c
+++ b/drivers/tty/serial/serial_core.c
@@ -2024,11 +2024,6 @@ static int uart_proc_show(struct seq_file *m, void *v)
 }
 #endif
 
-static inline bool uart_console_enabled(struct uart_port *port)
-{
-	return uart_console(port) && (port->cons->flags & CON_ENABLED);
-}
-
 static void uart_port_spin_lock_init(struct uart_port *port)
 {
 	spin_lock_init(&port->lock);
diff --git a/include/linux/serial_core.h b/include/linux/serial_core.h
index b7b86ee3cb12..9d8aa139b175 100644
--- a/include/linux/serial_core.h
+++ b/include/linux/serial_core.h
@@ -404,6 +404,11 @@ static const bool earlycon_acpi_spcr_enable EARLYCON_USED_OR_UNUSED;
 static inline int setup_earlycon(char *buf) { return 0; }
 #endif
 
+static inline bool uart_console_enabled(struct uart_port *port)
+{
+	return uart_console(port) && (port->cons->flags & CON_ENABLED);
+}
+
 struct uart_port *uart_get_console(struct uart_port *ports, int nr,
 				   struct console *c);
 int uart_parse_earlycon(char *p, unsigned char *iotype, resource_size_t *addr,

-- 
tg: (f287f971e256..) fix/console-usage_count (depends on: tty-next)