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

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Message-ID: <fcad430b-b9e8-4b1d-9dc9-195d12da5b00@gmail.com>
Date:   Wed, 4 Oct 2023 09:39:17 +0200
From:   Maximilian Luz <luzmaximilian@...il.com>
To:     Tony Lindgren <tony@...mide.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jirislaby@...nel.org>,
        Andy Shevchenko <andriy.shevchenko@...el.com>,
        Dhruva Gole <d-gole@...com>,
        Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>,
        John Ogness <john.ogness@...utronix.de>,
        Johan Hovold <johan@...nel.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Vignesh Raghavendra <vigneshr@...com>,
        linux-omap@...r.kernel.org,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        linux-kernel@...r.kernel.org, linux-serial@...r.kernel.org
Subject: Re: [PATCH v12 1/1] serial: core: Start managing serial controllers
 to enable runtime PM

On 10/4/23 08:17, Tony Lindgren wrote:
> * Maximilian Luz <luzmaximilian@...il.com> [231003 22:09]:
>> On 10/3/23 14:21, Tony Lindgren wrote:
>>> * Tony Lindgren <tony@...mide.com> [231003 12:15]:
>>>> Hi,
>>>>
>>>> * Maximilian Luz <luzmaximilian@...il.com> [231003 11:57]:
>>>>> A bad workaround is to disable runtime PM, e.g. via
>>>>>
>>>>>     echo on > /sys/bus/serial-base/devices/dw-apb-uart.4:0/dw-apb-uart.4:0.0/power/control
>>>>
>>>> If the touchscreen controller driver(s) are using serdev they are
>>>> children of the dw-apb-uart.4:0.0 and can use runtime PM calls to
>>>> block the parent device from idling as necessary. The hierarchy
>>>> unless changed using ignore_children.
>>>
>>> Sorry about all the typos, I meant "the hierarchy works unless changed"
>>> above. The rest of the typos are easier to decipher probably :)
>>
>> Unfortunately that doesn't quite line up with what I can see on v6.5.5. The
>> serdev controller seems to be a child of dw-apb-uart.4, a platform device. The
>> serial-base and serdev devices are siblings. According to sysfs:
>>
>>      /sys/bus/platform/devices/dw-apb-uart.4
>>      ├── driver -> ../../../../bus/platform/drivers/dw-apb-uart
>>      ├── subsystem -> ../../../../bus/platform
>>      │
>>      ├── dw-apb-uart.4:0
>>      │  ├── driver -> ../../../../../bus/serial-base/drivers/ctrl
>>      │  ├── subsystem -> ../../../../../bus/serial-base
>>      │  │
>>      │  └── dw-apb-uart.4:0.0
>>      │     ├── driver -> ../../../../../../bus/serial-base/drivers/port
>>      │     └── subsystem -> ../../../../../../bus/serial-base
>>      │
>>      └── serial0
>>         ├── subsystem -> ../../../../../bus/serial
>>         │
>>         └── serial0-0
>>            ├── driver -> ../../../../../../bus/serial/drivers/surface_serial_hub
>>            └── subsystem -> ../../../../../../bus/serial
> 
> The hierachy above is correct. Looks like I pasted the wrong device above,
> I meant dw-apb-uart.4, sorry about the extra confusion added. Eventually
> the serdev device could be a child of dw-apb-uart.4:0.0 at some point as
> it's specific to a serial port instance, but for now that should not be
> needed.
> 
> If serial0-0 is runtime PM active, then dw-apb-uart.4 is runtime PM active
> also unless ingore_children is set.
> 
>> Runtime suspend on serial0-0 is disabled/not set up at all. So I assume that if
>> it were a descendent of dw-apb-uart.4:0.0, things should have worked
>> out-of-the-box.
> 
> Hmm yes so maybe the issue is not with surface_serial_hub, but with serial
> port device being nable to resume after __device_suspend_late() has
> disabled runtime PM like you've been saying.
> 
> If the issue is with the serial port not being able to runtime resume, then
> the patch below should help. Care to give it a try?

Your patch does indeed make it work. So that's at least a better workaround
that we can carry in our downstream for now. Thanks!

Regards,
Max

> 8< ------------------
> diff --git a/drivers/tty/serial/serial_port.c b/drivers/tty/serial/serial_port.c
> --- a/drivers/tty/serial/serial_port.c
> +++ b/drivers/tty/serial/serial_port.c
> @@ -46,8 +46,27 @@ static int serial_port_runtime_resume(struct device *dev)
>   	return 0;
>   }
>   
> -static DEFINE_RUNTIME_DEV_PM_OPS(serial_port_pm,
> -				 NULL, serial_port_runtime_resume, NULL);
> +/*
> + * Allow serdev devices to talk to hardware during system suspend.
> + * Assumes the serial port hardware controller device driver calls
> + * pm_runtime_force_suspend() and pm_runtime_force_resume() for
> + * system suspend as needed.
> + */
> +static int serial_port_prepare(struct device *dev)
> +{
> +	return pm_runtime_resume_and_get(dev);
> +}
> +
> +static void serial_port_complete(struct device *dev)
> +{
> +	pm_runtime_put_sync(dev);
> +}
> +
> +static const struct dev_pm_ops __maybe_unused serial_port_pm = {
> +	SET_RUNTIME_PM_OPS(NULL, serial_port_runtime_resume, NULL)
> +	.prepare = serial_port_prepare,
> +	.complete = serial_port_complete,
> +};
>   
>   static int serial_port_probe(struct device *dev)
>   {