Re: serial 00:06: Runtime PM usage count underflow!

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

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On Sat, 16 Nov 2024, Borislav Petkov wrote:

> Hi folks,
> 
> got a box here which says:
> 
> [    4.654361] Serial: 8250/16550 driver, 4 ports, IRQ sharing enabled
> [    4.660820] 00:06: ttyS0 at I/O 0x3f8 (irq = 4, base_baud = 115200) is a 16550A
> [    4.668313] printk: legacy console [ttyS0] disabled
> [    4.676903] serial 00:06: Runtime PM usage count underflow!
> [    4.677175] serial 00:06: disabled
> [    4.677264] serial 00:06: unable to assign resources
> [    4.677356] serial 00:06: probe with driver serial failed with error -16
> [    4.677923] printk: legacy console [ttyS0] disabled
> 
> Any suggestions?
> 
> I'd like to use the serial cable to catch dmesg but somehow the driver doesn't
> like the chip...

I think "Runtime PM usage count underflow!" is not related to the probe 
problem you're seeing. Nonetheless, this patch might solve the underflow:

--
[PATCH 1/1] tty: serial: 8250: Fix another runtime PM usage counter underflow

The commit f9b11229b79c ("serial: 8250: Fix PM usage_count for console
handover") fixed one runtime PM usage counter balance problem that
occurs because .dev is not set during univ8250 setup preventing call to
pm_runtime_get_sync(). Later, univ8250_console_exit() will trigger the
runtime PM usage counter underflow as .dev is already set at that time.

Call pm_runtime_get_sync() to balance the RPM usage counter also in
serial8250_register_8250_port() before trying to add the port.

Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
---
 drivers/tty/serial/8250/8250_core.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/tty/serial/8250/8250_core.c b/drivers/tty/serial/8250/8250_core.c
index 5f9f06911795..68baf75bdadc 100644
--- a/drivers/tty/serial/8250/8250_core.c
+++ b/drivers/tty/serial/8250/8250_core.c
@@ -812,6 +812,9 @@ int serial8250_register_8250_port(const struct uart_8250_port *up)
 			uart->dl_write = up->dl_write;
 
 		if (uart->port.type != PORT_8250_CIR) {
+			if (uart_console_registered(&uart->port))
+				pm_runtime_get_sync(uart->port.dev);
+
 			if (serial8250_isa_config != NULL)
 				serial8250_isa_config(0, &uart->port,
 						&uart->capabilities);
-- 
2.39.5