Re: [PATCH 1/4] serial: core: Add wakeup() and start_pending_tx() for power management

[Tony Lindgren <tony@atomide.com>]

* Andy Shevchenko <andy.shevchenko@gmail.com> [210930 07:11]:
> On Thu, Sep 30, 2021 at 9:30 AM Tony Lindgren <tony@atomide.com> wrote:
> >
> > If the serial driver implements PM runtime with autosuspend, the port may
> > be powered down on TX. To wake up the port, let's add new wakeup() call
> > for serial drivers to implement as needed. We can call wakeup() from
> > __uart_start() and flow control related functions before attempting to
> > write to the serial port registers.
> >
> > Let's keep track of the serial port with a new runtime_suspended flag
> > that the device driver runtime PM suspend and resume can manage with
> > atomic_set(). This is because only the device driver knows what the
> > device runtime PM state as in Documentation/power/runtime_pm.rst
> > under "9. Autosuspend, or automatically-delayed suspend" for locking.
> >
> > To allow the serial port drivers to send out pending tx on runtime PM
> > resume, let's add start_pending_tx() as suggested by Johan Hovold
> > <johan@kernel.org>.
> 
> ...
> 
> > +  wakeup(port)
> > +       Wake up port if it has been runtime PM suspended.
> > +
> > +       Locking: port->lock taken.
> > +
> > +       Interrupts: locally disabled.
> 
> > +       This call must not sleep
> 
> If it's suspended via ACPI methods, it can't be resumed here, right?

It should work for that too assuming the runtime PM resume function is
implemented.

> Only what we can do is to schedule a resume, but it means we may not
> access registers immediately after and we have to be sure that the
> device is resumed.

Yeah the only thing we do in the 8250_port wakeup() is schedule a
resume if needed. Then the 8250 port device driver can call
start_pending_tx() at the end of it's runtime PM resume function.

> Dead end?

I don't think so :) In serial_core we bail out on uart_port_wakeup()
errors before register access. But maybe I missed some more moles to
whack there :)

Regards,

Tony