Re: [PATCH] serial: core: Fix checks for tx runtime PM state

[Andy Shevchenko <andriy.shevchenko@linux.intel.com>]

On Thu, Oct 05, 2023 at 10:56:42AM +0300, Tony Lindgren wrote:
> Maximilian reported that surface_serial_hub serdev tx does not work during
> system suspend. During system suspend, runtime PM gets disabled in
> __device_suspend_late(), and tx is unable to wake-up the serial core port
> device that we use to check if tx is safe to start. Johan summarized the
> regression noting that serdev tx no longer always works as earlier when the
> serdev device is runtime PM active.
> 
> The serdev device and the serial core controller devices are siblings of
> the serial port hardware device. The runtime PM usage count from serdev

I'm a bit lost in terminology here.
AFAIU there are:
1) children of the serial physical device;
2) siblings (to each other).

But may be I mistakenly deciphered the diagram from the previous discussion.

> device does not propagate to the serial core device siblings, it only
> propagates to the parent.
> 
> In addition to the tx issue for suspend, testing for the serial core port
> device can cause an unnecessary delay in enabling tx while waiting for the
> serial core port device to wake-up. The serial core port device wake-up is
> only needed to flush pending tx when the serial port hardware device was
> in runtime PM suspended state.
> 
> To fix the regression, we need to check the runtime PM state of the parent
> serial port hardware device for tx instead of the serial core port device.
> 
> As the serial port device drivers may or may not implement runtime PM, we
> need to also add a check for pm_runtime_enabled().

Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>

-- 
With Best Regards,
Andy Shevchenko