Re: [RFC PATCH] serial: core: Use pm_runtime_get_sync() in uart_start()

[Tony Lindgren]

* Xuewen Yan <xuewen.yan@xxxxxxxxxx> [231124 12:25]:
> The commit 84a9582fd203("serial: core: Start managing serial controllers to enable runtime PM")
> use the pm_runtime_get() after uart_port_lock() which would close the irq and disable preement.
> At this time,  pm_runtime_get may cause the following two problems:
> 
> (1) deadlock in try_to_wake_up:
> 
> uart_write()
> uart_port_lock() <<< get lock
> __uart_start
> __pm_runtime_resume
> rpm_resume
> queue_work_on
> try_to_wake_up
> _printk
> uart_console_write
> ...
> uart_port_lock() <<< wait forever
> 
> (2) scheduling while atomic:
> uart_write()
> uart_port_lock() <<< get lock
> __uart_start
> __pm_runtime_resume
> rpm_resume
> chedule() << sleep

Are these spinlock vs raw spinlock nesting warnings from lockdep? If so
can you please post the full warnings somewhere? Or if some extra steps
are needed to reproduce please describe that too.

Chances are very high that your serial port is already runtime active at
this point unless you manually idle it so that's why I'm wondering as
all that likely is happening here is a check on the runtime PM usage count.

> So let us use pm_runtime_get_sync() to prevent these.

We need to fix this some other way as we can't use pm_runtime_get_sync()
here. The sync call variants require setting pm_runtime_irq_safe() for the
device. And this is what we really want to avoid as it takes a permanent
usage count on the parent device.

What we want to do here is to get runtime PM to wake-up the device if idle
and try tx again after runtime PM resume as needed.

Just guessing at this point.. To me it sounds like the fix might be to
use a raw spinlock for uart_port_lock() and uart_port_unlock()?

Regards,

Tony


> Fixes: 84a9582fd203 ("serial: core: Start managing serial controllers to enable runtime PM")
> Signed-off-by: Xuewen Yan <xuewen.yan@xxxxxxxxxx>
> ---
>  drivers/tty/serial/serial_core.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
> index f1348a509552..902f7ed35f4d 100644
> --- a/drivers/tty/serial/serial_core.c
> +++ b/drivers/tty/serial/serial_core.c
> @@ -145,7 +145,7 @@ static void __uart_start(struct uart_state *state)
>  	port_dev = port->port_dev;
>  
>  	/* Increment the runtime PM usage count for the active check below */
> -	err = pm_runtime_get(&port_dev->dev);
> +	err = pm_runtime_get_sync(&port_dev->dev);
>  	if (err < 0 && err != -EINPROGRESS) {
>  		pm_runtime_put_noidle(&port_dev->dev);
>  		return;
> @@ -159,7 +159,7 @@ static void __uart_start(struct uart_state *state)
>  	if (!pm_runtime_enabled(port->dev) || pm_runtime_active(port->dev))
>  		port->ops->start_tx(port);
>  	pm_runtime_mark_last_busy(&port_dev->dev);
> -	pm_runtime_put_autosuspend(&port_dev->dev);
> +	pm_runtime_put_sync_autosuspend(&port_dev->dev);
>  }
>  
>  static void uart_start(struct tty_struct *tty)
> -- 
> 2.25.1
>