[PATCH v2 0/2] serial: 8250_port: Update runtime PM flow

[PATCH v2 0/2] serial: 8250_port: Update runtime PM flow

[Andy Shevchenko]

There are two changes, one is a straightforward drop of the duplicate
runtime PM call (which is idempotent and hence it's harmless to call,
but practically no need to do so) and the other addresses long standing
problem with potentially sleeping PM calls on some system in IRQ context.
Also the latter might lead to unneeded resume-suspend cycle when IRQ is
shared and interrupt is not ours. This mini-series to update runtime
PM flow to make sure this won't happen.

In v2:
- removed now unused local variable (Greg)

v1: <20260814112435.3290545-1-andriy.shevchenko@linux.intel.com>

Andy Shevchenko (2):
  serial: 8250_port: Remove redundant pm_runtime_mark_last_busy() call
  serial: 8250_port: properly handle runtime PM in IRQ

 drivers/tty/serial/8250/8250_port.c | 17 ++++++++++++-----
 1 file changed, 12 insertions(+), 5 deletions(-)

-- 
2.50.1

[PATCH v2 1/2] serial: 8250_port: Remove redundant pm_runtime_mark_last_busy() call

[Andy Shevchenko]

The pm_runtime_mark_last_busy() call is redundant in the wrapper function
as pm_runtime_put_autosuspend() already calls pm_runtime_mark_last_busy()
internally to update the last access time of the device before queuing
autosuspend.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
---
 drivers/tty/serial/8250/8250_port.c | 2 --
 1 file changed, 2 deletions(-)

diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
index 38fa45e74a37..c0bcc0742a60 100644
--- a/drivers/tty/serial/8250/8250_port.c
+++ b/drivers/tty/serial/8250/8250_port.c
@@ -526,7 +526,6 @@ void serial8250_rpm_put(struct uart_8250_port *p)
 {
 	if (!(p->capabilities & UART_CAP_RPM))
 		return;
-	pm_runtime_mark_last_busy(p->port.dev);
 	pm_runtime_put_autosuspend(p->port.dev);
 }
 EXPORT_SYMBOL_GPL(serial8250_rpm_put);
@@ -667,7 +666,6 @@ static void serial8250_rpm_put_tx(struct uart_8250_port *p)
 	rpm_active = xchg(&p->rpm_tx_active, 0);
 	if (!rpm_active)
 		return;
-	pm_runtime_mark_last_busy(p->port.dev);
 	pm_runtime_put_autosuspend(p->port.dev);
 }
 
-- 
2.50.1

[PATCH v2 2/2] serial: 8250_port: properly handle runtime PM in IRQ

[Andy Shevchenko]

We can't and basically don't need to call runtime PM in IRQ handler.
If IRQ is ours, the device must be powered on. Otherwise check if
the device is powered off and return immediately.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
---
 drivers/tty/serial/8250/8250_port.c | 15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
index c0bcc0742a60..1e62460b19ab 100644
--- a/drivers/tty/serial/8250/8250_port.c
+++ b/drivers/tty/serial/8250/8250_port.c
@@ -1878,13 +1878,22 @@ EXPORT_SYMBOL_GPL(serial8250_handle_irq);
 
 static int serial8250_default_handle_irq(struct uart_port *port)
 {
-	struct uart_8250_port *up = up_to_u8250p(port);
 	unsigned int iir;
+	int pm_status;
+	int ret;
 
-	guard(serial8250_rpm)(up);
+	/* if driver suspended, return, probably shared interrupt */
+	pm_status = pm_runtime_get_if_active(port->dev);
+	if (!pm_status)
+		return 0;
 
 	iir = serial_port_in(port, UART_IIR);
-	return serial8250_handle_irq(port, iir);
+	ret = serial8250_handle_irq(port, iir);
+
+	if (pm_status > 0)
+		pm_runtime_put_autosuspend(port->dev);
+
+	return ret;
 }
 
 /*
-- 
2.50.1

Re: [PATCH v2 1/2] serial: 8250_port: Remove redundant pm_runtime_mark_last_busy() call

[sashiko-bot]

> The pm_runtime_mark_last_busy() call is redundant in the wrapper function
> as pm_runtime_put_autosuspend() already calls pm_runtime_mark_last_busy()
> internally to update the last access time of the device before queuing
> autosuspend.
> 
> Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>

Sashiko has reviewed this patch and found no issues. It looks great!

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260924122549.668680-1-andriy.shevchenko@linux.intel.com?part=1

Re: [PATCH v2 2/2] serial: 8250_port: properly handle runtime PM in IRQ

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [High] serial: 8250: missing port->dev NULL check in serial8250_default_handle_irq()
--

commit 128116d52224b04215dd65b2f4f799e8c14bdec7
Author: Andy Shevchenko <andriy.shevchenko@linux.intel.com>

serial: 8250_port: properly handle runtime PM in IRQ

This commit updates the 8250 driver IRQ handler to explicitly check the
runtime PM status before processing the interrupt. It returns immediately
if the device is powered off, assuming it is a shared interrupt.

> diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
> index c0bcc0742a60c..1e62460b19ab9 100644
> --- a/drivers/tty/serial/8250/8250_port.c
> +++ b/drivers/tty/serial/8250/8250_port.c
> @@ -1878,13 +1878,22 @@ EXPORT_SYMBOL_GPL(serial8250_handle_irq);
>  
>  static int serial8250_default_handle_irq(struct uart_port *port)
>  {
[ ... ]
> +	/* if driver suspended, return, probably shared interrupt */
> +	pm_status = pm_runtime_get_if_active(port->dev);

[Severity: High]
Will this trigger a kernel panic if the port has no associated device? 
Some ports, such as legacy ISA serial ports, might have a NULL port->dev. 
Calling pm_runtime_get_if_active() with a NULL pointer would unconditionally 
dereference it.

> +	if (!pm_status)
> +		return 0;
>  
>  	iir = serial_port_in(port, UART_IIR);
> -	return serial8250_handle_irq(port, iir);
> +	ret = serial8250_handle_irq(port, iir);
> +
> +	if (pm_status > 0)
> +		pm_runtime_put_autosuspend(port->dev);

[Severity: High]
Similarly, does this put operation need a check to ensure port->dev is valid
before passing it to pm_runtime_put_autosuspend()?

> +
> +	return ret;
>  }

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260924122549.668680-1-andriy.shevchenko@linux.intel.com?part=2

Re: [PATCH v2 2/2] serial: 8250_port: properly handle runtime PM in IRQ

[Andy Shevchenko]

On Thu, Sep 24, 2026 at 12:40:41PM +0000, sashiko-bot@kernel.org wrote:
> Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
> - [High] serial: 8250: missing port->dev NULL check in serial8250_default_handle_irq()
> --

Isn't this a preexisting issue since serial8250_rpm_get() is implemented this way
https://elixir.bootlin.com/linux/v7.3-rc3/source/drivers/tty/serial/8250/8250_port.c#L517
?

Yes, only for platforms with UART_CAP_RPM, but it doesn't matter, they have no
special handling for the port in this case. (And pm_runtime_get_if_active() actually
works properly on those which do not set that flag as we only check pm_status for 0
and don't care for the other cases.)

> commit 128116d52224b04215dd65b2f4f799e8c14bdec7
> Author: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
> 
> serial: 8250_port: properly handle runtime PM in IRQ
> 
> This commit updates the 8250 driver IRQ handler to explicitly check the
> runtime PM status before processing the interrupt. It returns immediately
> if the device is powered off, assuming it is a shared interrupt.
> 
> > diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
> > index c0bcc0742a60c..1e62460b19ab9 100644
> > --- a/drivers/tty/serial/8250/8250_port.c
> > +++ b/drivers/tty/serial/8250/8250_port.c
> > @@ -1878,13 +1878,22 @@ EXPORT_SYMBOL_GPL(serial8250_handle_irq);
> >  
> >  static int serial8250_default_handle_irq(struct uart_port *port)
> >  {
> [ ... ]
> > +	/* if driver suspended, return, probably shared interrupt */
> > +	pm_status = pm_runtime_get_if_active(port->dev);
> 
> [Severity: High]
> Will this trigger a kernel panic if the port has no associated device? 
> Some ports, such as legacy ISA serial ports, might have a NULL port->dev. 
> Calling pm_runtime_get_if_active() with a NULL pointer would unconditionally 
> dereference it.
> 
> > +	if (!pm_status)
> > +		return 0;
> >  
> >  	iir = serial_port_in(port, UART_IIR);
> > -	return serial8250_handle_irq(port, iir);
> > +	ret = serial8250_handle_irq(port, iir);
> > +
> > +	if (pm_status > 0)
> > +		pm_runtime_put_autosuspend(port->dev);
> 
> [Severity: High]
> Similarly, does this put operation need a check to ensure port->dev is valid
> before passing it to pm_runtime_put_autosuspend()?

Same as above. We check pm_status. It can't be positive if above case failed
already.

> > +	return ret;
> >  }

-- 
With Best Regards,
Andy Shevchenko