[PATCH 2/3] dt-bindings: watchdog: fsl-imx: document suspend in wait mode

[Andrej Picej]

On 20. 10. 22 14:41, Alexander Stein wrote:
> Am Donnerstag, 20. Oktober 2022, 14:36:10 CEST schrieb Andrej Picej:
>> On 20. 10. 22 14:18, Krzysztof Kozlowski wrote:
>>> On 20/10/2022 02:23, Andrej Picej wrote:
>>>> Hi Alexander and Krzysztof,
>>>>
>>>> hope I can reply to both questions here.
>>>>
>>>> On 19. 10. 22 17:51, Krzysztof Kozlowski wrote:
>>>>> On 19/10/2022 09:00, Alexander Stein wrote:
>>>>>> Hello Andrej,
>>>>>
>>>>>> Am Mittwoch, 19. Oktober 2022, 13:17:13 CEST schrieb Andrej Picej:
>>>>> Missing commit msg.
>>>>>
>>>>>>> Signed-off-by: Andrej Picej <andrej.picej at norik.com>
>>>>>>> ---
>>>>>>>
>>>>>>>     Documentation/devicetree/bindings/watchdog/fsl-imx-wdt.yaml | 5
>>>>>>>     +++++
>>>>>>>     1 file changed, 5 insertions(+)
>>>>>>>
>>>>>>> diff --git
>>>>>>> a/Documentation/devicetree/bindings/watchdog/fsl-imx-wdt.yaml
>>>>>>> b/Documentation/devicetree/bindings/watchdog/fsl-imx-wdt.yaml index
>>>>>>> fb7695515be1..01b3e04e7e65 100644
>>>>>>> --- a/Documentation/devicetree/bindings/watchdog/fsl-imx-wdt.yaml
>>>>>>> +++ b/Documentation/devicetree/bindings/watchdog/fsl-imx-wdt.yaml
>>>>>>>
>>>>>>> @@ -55,6 +55,11 @@ properties:
>>>>>>>           If present, the watchdog device is configured to assert its
>>>>>>>           external reset (WDOG_B) instead of issuing a software reset.
>>>>>>>
>>>>>>> +  fsl,suspend-in-wait:
>>>>>>> +    $ref: /schemas/types.yaml#/definitions/flag
>>>>>>> +    description: |
>>>>>>> +      If present, the watchdog device is suspended in WAIT mode.
>>>>>>> +
>>>>>>>
>>>>>>>     required:
>>>>>>>       - compatible
>>>>>>>       - interrupts
>>>>>>
>>>>>> What is the condition the watchdog is suspended in WAIT mode? Is this
>>>>>> specific to SoC or platform or something else?
>>>>
>>>> Sorry, what exactly do you mean by condition?
>>>
>>> Ugh, I also cannot parse it now...
> 
> Sorry, Krzysztof already asked the right question: When does one want to
> enable/disable this feature?
> 
>>>> When the property
>>>> "fsl,suspend-in-wait" is set the watchdog is suspended in WAIT mode, so
>>>> this is defined by the user. Didn't want to apply it for all the
>>>> supported machines since there could be devices which depend on watchdog
>>>> triggering in WAIT mode. We stumbled on this problem on imx6 devices,
>>>> but the same bit (with the same description) is found on imx25, imx35,
>>>> imx50/51/53, imx7 and imx8.
>>>
>>> I meant, what is expected to happen if you do not enable this bit and
>>> watchdog triggers in WAIT mode? IOW, why someone might want to enable or
>>> disable this property?
>>
>> If this is not enabled and you put the device into the Suspend-to-idle
>> mode the device resets after 128 seconds. If not, the device can be left
>> in that state for infinite time. I'm guessing you want me to better
>> explain the property in device tree docs right?
>> I can do that in v2.
>>
>>>>> And what happens else? When it is not suspended in WAIT mode?
>>>>
>>>> When you put the device in "freeze"/"Suspend-To-Idle" low-power mode the
>>>> watchdog keeps running and triggers a reset after 128 seconds. So the
>>>> maximum length the device can stay in this mode is limited to 128
>>>> seconds.
>>>
>>> And who wakes up the system before 128 seconds? IOW is there a use case
>>> of not enabling this property?
>>
>> Well I can think of one, system can be woken up by some other interrupt.
>> Like RTC which triggers interrupt (for example every 10s). So if this
>> property is left disabled the watchdog can handle errors where other
>> wakeup sources don't trigger interrupt or if the system is unable to
>> wake from low-power state. In that case the watchdog will do a hard
>> reset of the device.
>>
>> But I'm not really sure if anybody uses this, just wanted to make sure
>> that we keep the default behaviour as it is, since this driver is used
>> by many devices and for quite some time.
> 
> This sounds more like (application) configuration. If so this should not be
> configured in device tree, IMHO.
> 

Do you have an idea where should it be configured? Just keep in mind 
that this can not be configured at runtime, since this is write-once bit 
so any configuration changes regarding this functionality can not be done.

Basically if I can sum up the problem:

Without this property enabled, the WDW bit is left unset:
$ echo freeze > /sys/power/state
#device enters Suspend-to-idle, watchdog is left running and the device 
resets after 128 seconds in this state

With this property set, the WDW bit is set at watchdog initialization:
$ echo freeze > /sys/power/state
#device enters Suspend-to-idle, watchdog is suspended and the device can 
be left in this state until some other wakeup source triggers interrupt.

Thanks,
Andrej