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From:   Andy Shevchenko <andy.shevchenko@gmail.com>
Date:   Thu, 30 Sep 2021 10:10:49 +0300
Message-ID: <CAHp75Ve4RTSdbQYA_u8vs=U75KsNdrm9EqFASAGf4rFKSqVWvQ@mail.gmail.com>
Subject: Re: [PATCH 1/4] serial: core: Add wakeup() and start_pending_tx() for
 power management
To:     Tony Lindgren <tony@atomide.com>
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Andy Shevchenko <andriy.shevchenko@intel.com>,
        Jiri Slaby <jirislaby@kernel.org>,
        Johan Hovold <johan@kernel.org>,
        Vignesh Raghavendra <vigneshr@ti.com>,
        linux-serial@vger.kernel.org, linux-omap@vger.kernel.org,
        linux-kernel@vger.kernel.org
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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?
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.

Dead end?

-- 
With Best Regards,
Andy Shevchenko