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

[Maximilian Luz <luzmaximilian@gmail.com>]

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

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.

Regards,
Max