Re: [PATCH 4/6] serial: 8250: Implement prep_tx for power management

[Tony Lindgren]

* Johan Hovold <johan@xxxxxxxxxx> [210924 14:44]:
> On Thu, Sep 23, 2021 at 06:05:36PM +0300, Tony Lindgren wrote:
> > * Johan Hovold <johan@xxxxxxxxxx> [210923 12:50]:
> > > On Tue, Sep 21, 2021 at 01:33:44PM +0300, Tony Lindgren wrote:
> > > > +static int serial8250_prep_tx(struct uart_port *port)
> > > > +{
> > > > +	struct uart_8250_port *up = up_to_u8250p(port);
> > > > +	struct device *dev = up->port.dev;
> > > > +	int err;
> > > > +
> > > > +	if (!(up->capabilities & UART_CAP_RPM))
> > > > +		return 0;
> > > > +
> > > > +	if (!pm_runtime_suspended(dev)) {
> > > > +		pm_runtime_mark_last_busy(dev);
> > > > +		return 0;
> > > > +	}
> > > > +
> > > > +	err = pm_request_resume(dev);
> > > > +	if (err < 0) {
> > > > +		dev_warn(dev, "prep_tx wakeup failed: %d\n", err);
> > > > +		return err;
> > > > +	}
> > > 
> > > How is this supposed to work without a runtime PM usage-counter
> > > increment? What's to prevent the port from suspending again while it's
> > > transmitting?
> > 
> > Hmm yeah we should at pm_runtime_get() and pm_runtime_put() to write()
> > unless serial8250_rpm_get() and serial8250_rpm_put() are doing it.
> 
> If you do the put after just buffering the data it doesn't really solve
> anything.

Right, sounds like we currently rely on the autosuspend_timeout
there.

> > Or pair prep with finish and deal with the usage count there.
> 
> Problem is where to call it from. How do you tell the device is done
> transmitting? And how should we deal with flow control? Etc.

Maybe if the device driver needs to call uart_start() also from runtime
PM idle function and if no data allow suspend. Then if there is
more data, uart_write() calls uart_start() again, device wakes up
and so on.

Regards,

Tony