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SubjectRe: [PATCH v12 1/1] serial: core: Start managing serial controllers to enable runtime PM
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On 10/4/23 08:17, Tony Lindgren wrote:
> * Maximilian Luz <luzmaximilian@gmail.com> [231003 22:09]:
>> On 10/3/23 14:21, Tony Lindgren wrote:
>>> * Tony Lindgren <tony@atomide.com> [231003 12:15]:
>>>> Hi,
>>>>
>>>> * Maximilian Luz <luzmaximilian@gmail.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)
> {

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