linux-kernel - Re: [PATCH 1/2] dt-bindings: watchdog: intel: Add YAML Schemas for Watchdog timer

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Message-ID: <83812b78-41e3-299d-36a0-6ce1576e7e78@roeck-us.net>
Date:   Tue, 9 Jun 2020 06:46:15 -0700
From:   Guenter Roeck <linux@...ck-us.net>
To:     Dilip Kota <eswara.kota@...ux.intel.com>, wim@...ux-watchdog.org,
        linux-watchdog@...r.kernel.org, devicetree@...r.kernel.org,
        robbh@...nel.org
Cc:     linux-kernel@...r.kernel.org, andriy.shevchenko@...el.com,
        cheol.yong.kim@...el.com, qi-ming.wu@...el.com, yixin.zhu@...el.com
Subject: Re: [PATCH 1/2] dt-bindings: watchdog: intel: Add YAML Schemas for
 Watchdog timer

On 6/9/20 1:57 AM, Dilip Kota wrote:
> 
> On 6/8/2020 9:37 PM, Guenter Roeck wrote:
>> On 6/7/20 10:49 PM, Dilip Kota wrote:
>>> Add YAML schemas for the watchdog timer on Intel Lightning
>>> Mountain SoC.
>>>
>>> Signed-off-by: Dilip Kota <eswara.kota@...ux.intel.com>
>>> ---
>>>   .../bindings/watchdog/intel,lgm-gptc-wdt.yaml      | 75 ++++++++++++++++++++++
>>>   1 file changed, 75 insertions(+)
>>>   create mode 100644 Documentation/devicetree/bindings/watchdog/intel,lgm-gptc-wdt.yaml
>>>
>>> diff --git a/Documentation/devicetree/bindings/watchdog/intel,lgm-gptc-wdt.yaml b/Documentation/devicetree/bindings/watchdog/intel,lgm-gptc-wdt.yaml
>>> new file mode 100644
>>> index 0000000000000..83dc39a5090c1
>>> --- /dev/null
>>> +++ b/Documentation/devicetree/bindings/watchdog/intel,lgm-gptc-wdt.yaml
>>> @@ -0,0 +1,75 @@
>>> +# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
>>> +%YAML 1.2
>>> +---
>>> +$id: http://devicetree.org/schemas/watchdog/intel,lgm-gptc-wdt.yaml#
>>> +$schema: http://devicetree.org/meta-schemas/core.yaml#
>>> +
>>> +title: Intel Lightning Mountain Watchdog timer.
>>> +
>>> +maintainers:
>>> +  - Dilip Kota <eswara.kota@...ux.intel.com>
>>> +
>>> +description: |
>>> +  Intel Lightning Mountain SoC has General Purpose Timer Counter(GPTC) which can
>>> +  be configured as Clocksource, real time clock and Watchdog timer.
>>> +  Each General Purpose Timer Counter has three timers. And total four General
>>> +  Purpose Timer Counters are present on Lightning Mountain SoC which sums up
>>> +  to 12 timers.
>>> +  Lightning Mountain has four CPUs and each CPU is configured with one GPTC
>>> +  timer as watchdog timer. Total four timers are configured as watchdog timers
>>> +  on Lightning Mountain SoC.
>>> +
>> Why not just one ? The watchdog subsystem does not monitor individual CPUs,
>> it monitors the system.
> 
> Intel Atom based Lightning Mountain SoC, system has four CPUs. On Lightning Mountain SoC ,Watchdog subsystem is combination of GPTC timers and reset controller unit. On Lightning Mountain SoC, each CPU is configured with one GPTC timer, so that if any of the CPU hangs or freezes, the watchdog daemon running on respective CPU cannot reset/ping or pet the watchdog timer. This causes the watchdog timeout. On watchdog timeout, reset controller triggers the reset to respective CPU.
> 
A system watchdog driver should not duplicate functionality
from kernel/watchdog.c, which monitors individual CPUs.
If the SoC does nto provide a system watchdog timer (which
I think is unlikely), it should stick with that. A watchdog
resetting an individual CPU instead of the entire system
isn't something I would want to see in the watchdog subsystem.

Guenter

> 
> ____________________
> ----------------------------->|                |
>                                                    |         ------------>| Reset controller unit    |
> |                     |                 |___________________|
> |                     |
> |                     |
>   ______________________|__________|______
>   |                                  GPTC   | |            |
>   | ___________   ________|_       ______|____   |
>   | |      timer 1 |   | timer 2      |    | timer 3      |    |
>   | |_________|   |_________|    |_________|    |
>   |______________________________________|
> 
> Regards,
> -Dilip
> 
> 
>> Guenter
>>
>>