linux-kernel - Re: [PATCH 2/4] serial: 8250: Implement wakeup for TX and use it for 8250

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Message-ID: <CAHp75VeZ98Se+BBDdMeJmwu39CbXEL08RF4BR+uu5oJAycEb=A@mail.gmail.com>
Date:   Thu, 30 Sep 2021 10:17:21 +0300
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     Tony Lindgren <tony@...mide.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andy Shevchenko <andriy.shevchenko@...el.com>,
        Jiri Slaby <jirislaby@...nel.org>,
        Johan Hovold <johan@...nel.org>,
        Vignesh Raghavendra <vigneshr@...com>,
        "open list:SERIAL DRIVERS" <linux-serial@...r.kernel.org>,
        Linux OMAP Mailing List <linux-omap@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/4] serial: 8250: Implement wakeup for TX and use it for 8250_omap

On Thu, Sep 30, 2021 at 9:30 AM Tony Lindgren <tony@...mide.com> wrote:
>
> We can use the wakeup() and uart_start_pending_tx() calls to wake up an
> idle serial port and send out the pending TX buffer on runtime PM resume.

> This allows us to remove the depedency to pm_runtime_irq_safe() for

dependency

> 8250_omap driver in the following patches.
>
> We manage the port runtime_suspended flag in the serial port driver as
> only the driver knows when the hardware is runtime PM suspended. Note that
> The current flag for rpm_tx_active cannot be used as it is TX specific
> for 8250_port.
>
> We already have serial8250_start_tx() call serial8250_rpm_get_tx(), and
> serial8250_stop_tx() call serial8250_rpm_put_tx() to take care of the
> runtime PM usage count for TX. To have the serial port driver call
> uart_start_pending_tx() on runtime resume, we must now use just
> pm_runtime_get() for serial8250_start_tx() instead of the sync version.
>
> With these changes we must now also flip 8250_omap driver over to call
> uart_start_pending_tx(). That's currently the only user of UART_CAP_RPM.

Do I understand the flow correctly:
 1) if we suspended, we request resume
 2) until resume is not fulfilled we return error code to user space
to try again
?

In this case we have no register access to the powered off device and
ACPI, for example, may have a chance to resume the device in a
non-atomic way. Is this the correct interpretation?

-- 
With Best Regards,
Andy Shevchenko