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

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Date:   Thu, 20 Oct 2022 14:36:10 +0200
From:   Andrej Picej <andrej.picej@...ik.com>
To:     Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>,
        Alexander Stein <alexander.stein@...tq-group.com>
Cc:     linux-watchdog@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, shawnguo@...nel.org,
        linux@...ck-us.net, linux-kernel@...r.kernel.org,
        devicetree@...r.kernel.org, linux-imx@....com, festevam@...il.com,
        kernel@...gutronix.de, s.hauer@...gutronix.de,
        wim@...ux-watchdog.org, robh+dt@...nel.org
Subject: Re: [PATCH 2/3] dt-bindings: watchdog: fsl-imx: document suspend in
 wait mode



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@...ik.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...
> 
>> 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.

Best regards,
Andrej