Re: [PATCH v3 2/6] serial: core: Allow detach and attach serial device for console

[Tony Lindgren <tony@atomide.com>]

Hi,

* Geert Uytterhoeven <geert@linux-m68k.org> [200702 14:50]:
> On Sun, May 24, 2020 at 7:11 PM Guenter Roeck <linux@roeck-us.net> wrote:
> > any special reason for this change ? It is not really explained in the
> > commit description.
> 
> Indeed. Why this change?

For a kernel console, we want it to work for important oopses
etc without trying to enable DMA or power on regulators for example.

And we want to get rid of pm_runtime_irq_safe() usage as that takes
a permanent usage count on the parent device. This causes issue where
uart can keep an interconnect instance from idling with let's say
genpd :)

Also, we cannot easily make the serial driver PM runtime generic
without removing the pm_runtime_irq_safe() dependency as it would
currently wrongly impose the same nasty dependency to all the serial
drivers.

So when a kernel console is attached, we want to keep it from idling.

For PM related testing, just detaching kernel console and configuring
it's autosuspend timeout works nicely with dmesg -w. Or actually in
that case using pstore console is even better.

Andy has a series pending for generic serial PM runtime support,
I guess he's going to be posting it as soon as the pending regressions
are dealt with.

Regards,

Tony