Message-ID: <CA+V-a8tOFqt79D9K9vQsKYaou76b-=+73+4Y3RNc86uMXnqU5Q@mail.gmail.com>
Date: Mon, 4 Aug 2025 12:24:04 +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 Sun, Aug 3, 2025 at 1:16 AM Guenter Roeck <linux@...ck-us.net> wrote:
>
> On 8/2/25 12:26, Lad, Prabhakar wrote:
> > Hi Guenter,
> >
> > On Fri, Aug 1, 2025 at 10:04 PM Guenter Roeck <linux@...ck-us.net> wrote:
> >>
> >> On 8/1/25 13:51, Lad, Prabhakar wrote:
> >>> 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.
> >>>
> >>
> >> Just four possible values to set the hardware timeout ? That is an odd
> >> hardware. In that case, you could also set the period to 1024 or 4096
> >> and set max_hw_heartbeat_ms accordingly. That would avoid the rounding
> >> error.
> >>
> > Yes sadly we have four timeout periods only. To clarify, you mean to
> > set `max_hw_heartbeat_ms` in set_timeout?
> >
>
> No, during initialization, and have no set_timeout function. max_hw_heartbeat_ms
> is not supposed to change during runtime. If you do change it, the results
> are undefined.
>
Thank you for the clarification. Ive done the changes as suggested. I
will send a v3 soon.

Cheers,
Prabhakar

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