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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <c06bcde9-0aa5-46d1-a5bf-bae5a319565c@roeck-us.net>
Date: Fri, 1 Aug 2025 06:52:01 -0700
From: Guenter Roeck <linux@...ck-us.net>
To: "Lad, Prabhakar" <prabhakar.csengg@...il.com>,
 Wolfram Sang <wsa+renesas@...g-engineering.com>
Cc: 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 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 ?

Guenter