linux-kernel - Re: [PATCH v2 7/9] watchdog: rzv2h: Set min_timeout based on max_hw_heartbeat

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Message-ID: <cd0653d0-4a2f-4361-8eb2-c1937d988a8c@roeck-us.net>
Date: Fri, 1 Aug 2025 11:04:20 -0700
From: Guenter Roeck <linux@...ck-us.net>
To: "Lad, Prabhakar" <prabhakar.csengg@...il.com>
Cc: Wolfram Sang <wsa+renesas@...g-engineering.com>,
 Wim Van Sebroeck <wim@...ux-watchdog.org>, Rob Herring <robh@...nel.org>,
 Krzysztof Kozlowski <krzk+dt@...nel.org>, Conor Dooley
 <conor+dt@...nel.org>, Philipp Zabel <p.zabel@...gutronix.de>,
 Geert Uytterhoeven <geert+renesas@...der.be>,
 Magnus Damm <magnus.damm@...il.com>, linux-watchdog@...r.kernel.org,
 devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-renesas-soc@...r.kernel.org, Biju Das <biju.das.jz@...renesas.com>,
 Fabrizio Castro <fabrizio.castro.jz@...esas.com>,
 Lad Prabhakar <prabhakar.mahadev-lad.rj@...renesas.com>
Subject: Re: [PATCH v2 7/9] watchdog: rzv2h: Set min_timeout based on
 max_hw_heartbeat_ms

On 8/1/25 08:30, Lad, Prabhakar wrote:
> Hi Guenter,
> 
> On Fri, Aug 1, 2025 at 2:52 PM Guenter Roeck <linux@...ck-us.net> wrote:
>>
>> On 8/1/25 04:05, Lad, Prabhakar wrote:
>>> Hi Wolfram,
>>>
>>> Thank you for the review.
>>>
>>> On Fri, Aug 1, 2025 at 5:10 AM Wolfram Sang
>>> <wsa+renesas@...g-engineering.com> wrote:
>>>>
>>>> On Tue, Jul 29, 2025 at 04:59:13PM +0100, Prabhakar wrote:
>>>>> From: Lad Prabhakar <prabhakar.mahadev-lad.rj@...renesas.com>
>>>>>
>>>>> Update the watchdog minimum timeout value to be derived from
>>>>> `max_hw_heartbeat_ms` using `DIV_ROUND_UP()` to ensure a valid and
>>>>> consistent minimum timeout in seconds.
>>>>
>>>> I don't understand this change. Why is the _minimum_ timeout based on
>>>> the _maximum_ heartbeat?
>>>>
>>> The reason for deriving min_timeout from max_hw_heartbeat_ms is to
>>> ensure the minimum watchdog period (in seconds) is compatible with the
>>> underlying hardware.
>>>
>>> max_hw_heartbeat_ms is calculated as:
>>> max_hw_heartbeat_ms = (1000 * 16384 * cks_div) / clk_rate;
>>>
>>> This value varies by SoC:
>>>    RZ/T2H: cks_div = 8192, clk ≈ 62.5 MHz -> max_hw_heartbeat_ms ~ 2147ms
>>>    RZ/V2H: cks_div = 256, clk ≈ 240 MHz -> max_hw_heartbeat_ms ~ 174ms
>>>
>>> Since min_timeout is in seconds, setting it to:
>>> min_timeout = DIV_ROUND_UP(max_hw_heartbeat_ms, 1000);
>>>
>>> ensures:
>>> The minimum timeout period is never less than what the hardware can support.
>>> - For T2H, this results in a min_timeout of 3s (2147ms -> 3s).
>>> - For V2H, it’s just 1s (174ms -> 1s).
>>>
>>
>> Sorry, I completely fail to understand the logic.
>>
>> If the maximum timeout is, say, 2 seconds, why would the hardware
>> not be able to support a timeout of 1 second ?
>>
> The watchdog timer on RZ/V2H (and RZ/T2H) is a 14 bit down counter. On
> initialization the down counters on the SoCs are configured to the max
> down counter. On RZ/V2H down counter value 4194304 (which evaluates to
> 174ms) is and on RZ/T2H is 134217728 (which evaluates to 2147ms). The
> board will be reset when we get an underflow error.
> 
> So for example on T2H consider this example:
> - down counter is 134217728
> - min_timeout is set to 1 in the driver
> - When set  WDIOC_SETTIMEOUT to 1
> In this case the board will be reset after 2147ms, i.e. incorrect
> behaviour as we expect the board to be reset after 1 sec. Hence the
> min_timeout is set to 3s (2147ms -> 3s).
> 
> Please let me know if my understanding of min_timeout is incorrect here.
> 

The driver is missing a set_timeout function. It should set RZ/T2H
to 62514079 if a timeout of 1 second is configured.

Guenter