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

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Message-ID: <CA+V-a8txrQoweVrd7uK4LLvDonqrEQGT_gV1r28RFhy8-m=9VQ@mail.gmail.com>
Date: Fri, 1 Aug 2025 12:05:35 +0100
From: "Lad, Prabhakar" <prabhakar.csengg@...il.com>
To: Wolfram Sang <wsa+renesas@...g-engineering.com>
Cc: Wim Van Sebroeck <wim@...ux-watchdog.org>, Guenter Roeck <linux@...ck-us.net>, 
	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

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).

Cheers,
Prabhakar