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-a8v0KZaeJwJAmEpRRdS3F3vC_CYv7zGN_n9a+M6qhFDMHg@mail.gmail.com>
Date: Fri, 1 Aug 2025 21:51:06 +0100
From: "Lad, Prabhakar" <prabhakar.csengg@...il.com>
To: Guenter Roeck <linux@...ck-us.net>
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

Hi Guenter,

On Fri, Aug 1, 2025 at 7:04 PM Guenter Roeck <linux@...ck-us.net> wrote:
>
> 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.
>
Ok, you mean to handle the 1sec case, introduce the set_timeout for RZ/T2H SoC.

Although we cannot achieve the exact 1sec case as we can have only 4
timeout period options (number of cycles):

1] For TIMEOUT_CYCLES = 1024
 - (1000×1024×8192)/62500000 = 134.22 ms
2] For TIMEOUT_CYCLES = 4096
- (1000×4096×8192)/62500000 = 536.87 ms
3] For TIMEOUT_CYCLES = 8192
- (1000×8192×8192)/62500000 = 1,073.74 ms
4] For TIMEOUT_CYCLES = 16384
- (1000×16384×8192)/62500000 = 2,147.48 ms

So to handle the 1sec case I'll set the timeout period to 8192 with
which we get a timeout of 1,073.74 ms.

Cheers,
Prabhakar