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

[Tony Lindgren]

* 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.

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?

> 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.

Regards,

Tony