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From:   Chen-Yu Tsai <wenst@chromium.org>
Date:   Mon, 5 Jun 2023 21:01:29 +0800
Message-ID: <CAGXv+5HwL+R5QpO3pHGQd9qAxu2pCMDjYvdni1HjiC8eEE38mg@mail.gmail.com>
Subject: Re: [PATCH v12 1/1] serial: core: Start managing serial controllers
 to enable runtime PM
To:     Tony Lindgren <tony@atomide.com>
Cc:     John Ogness <john.ogness@linutronix.de>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Jiri Slaby <jirislaby@kernel.org>,
        Andy Shevchenko <andriy.shevchenko@intel.com>,
        Dhruva Gole <d-gole@ti.com>,
        =?UTF-8?Q?Ilpo_J=C3=A4rvinen?= <ilpo.jarvinen@linux.intel.com>,
        Johan Hovold <johan@kernel.org>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        Vignesh Raghavendra <vigneshr@ti.com>,
        linux-omap@vger.kernel.org,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        linux-kernel@vger.kernel.org, linux-serial@vger.kernel.org,
        =?UTF-8?B?TsOtY29sYXMgRi4gUi4gQS4gUHJhZG8=?= 
        <nfraprado@collabora.com>,
        AngeloGioacchino Del Regno 
        <angelogioacchino.delregno@collabora.com>,
        linux-mediatek@lists.infradead.org
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On Mon, Jun 5, 2023 at 8:24=E2=80=AFPM Tony Lindgren <tony@atomide.com> wro=
te:
>
> * Chen-Yu Tsai <wenst@chromium.org> [230605 11:34]:
> > On Mon, Jun 5, 2023 at 2:15=E2=80=AFPM Tony Lindgren <tony@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, str=
uct ktermios *termios,
> > >  static int __maybe_unused mtk8250_runtime_suspend(struct device *dev=
)
> > >  {
> > >         struct mtk8250_data *data =3D dev_get_drvdata(dev);
> > > -       struct uart_8250_port *up =3D 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 =3D 240, base_baud =
=3D
> > 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 th=
e
> > UART-based Bluetooth on one of my devices. After the UART runtime suspe=
nds
> > 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