[PATCH v12 1/1] serial: core: Start managing serial controllers to enable runtime PM

[Chen-Yu Tsai]

On Mon, Jun 5, 2023 at 8:24 PM Tony Lindgren <tony at atomide.com> wrote:
>
> * Chen-Yu Tsai <wenst at chromium.org> [230605 11:34]:
> > On Mon, Jun 5, 2023 at 2:15 PM Tony Lindgren <tony at atomide.com> wrote:
> > > diff --git a/drivers/tty/serial/8250/8250_mtk.c b/drivers/tty/serial/8250/8250_mtk.c
> > > --- a/drivers/tty/serial/8250/8250_mtk.c
> > > +++ b/drivers/tty/serial/8250/8250_mtk.c
> > > @@ -425,11 +439,10 @@ mtk8250_set_termios(struct uart_port *port, struct ktermios *termios,
> > >  static int __maybe_unused mtk8250_runtime_suspend(struct device *dev)
> > >  {
> > >         struct mtk8250_data *data = dev_get_drvdata(dev);
> > > -       struct uart_8250_port *up = serial8250_get_port(data->line);
> > >
> > >         /* wait until UART in idle status */
> > >         while
> > > -               (serial_in(up, MTK_UART_DEBUG0));
> > > +               (mtk8250_read(data, MTK_UART_DEBUG0));
> >
> > I believe it still gets stuck here sometimes.
>
> Hmm so maybe you need to mtk8250_write(data, 0, MTK_UART_RATE_FIX) in
> probe before pm_runtime_resume_and_get() that enables the baud clock?
> That's something I changed, so maybe it messes up things.

I think it has something to do with the do_pm() function calling
the callbacks directly, then also calling runtime PM.

> Looking at the 8250_mtk git log, it's runtime PM functions seem to only
> currently manage the baud clock so register access should be doable
> without runtime PM resume?

Actually it only manages the bus clock. The baud clock is simply the system
XTAL which is not gateble.

> > With your earlier patch, it could get through registering the port, and
> > the console would show
> >
> >     11002000.serial: ttyS0 at MMIO 0x11002000 (irq = 240, base_baud =
> > 1625000) is a ST16650V2
> >
> > for the console UART.
>
> OK
>
> > Angelo mentioned that we should be using SLEEP_REQ/SLEEP_ACK registers
> > in the MTK UART hardware.
> >
> > I tried reworking it into your patch here, but it causes issues with the
> > UART-based Bluetooth on one of my devices. After the UART runtime suspends
> > and resumes, something is off and causes the transfers during Bluetooth
> > init to become corrupt.
> >
> > I'll try some more stuff, but the existing code seems timing dependent.
> > If I add too many printk statements to the runtime suspend/resume
> > callbacks, things seem to work. One time I even ended up with broken
> > UARTs but otherwise booted up the system.
>
> Well another thing that now changes is that we now runtime suspend the
> port at the end of the probe. What the 8250_mtk probe was doing earlier
> it was leaving the port baud clock enabled, but runtime PM disabled
> until mtk8250_do_pm() I guess.

I guess that's the biggest difference? Since the *bus* clock gets disabled,
any access will hang. Is it enough to just support runtime PM? Or do I have
to also have UART_CAP_RPM?

ChenYu