Re: [PATCH v2 7/9] watchdog: rzv2h: Set min_timeout based on max_hw_heartbeat_ms

Guenter Roeck
2025-08-01T13:52:04Z

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@sang-engineering.com> wrote:
>>
>> On Tue, Jul 29, 2025 at 04:59:13PM +0100, Prabhakar wrote:
>>> From: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.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