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

On 8/2/25 12:26, Lad, Prabhakar wrote:

Hi Guenter,

On Fri, Aug 1, 2025 at 10:04 PM Guenter Roeck [off-list ref] wrote:

quoted

On 8/1/25 13:51, Lad, Prabhakar wrote:

quoted

Hi Guenter,

On Fri, Aug 1, 2025 at 7:04 PM Guenter Roeck [off-list ref] wrote:

quoted

On 8/1/25 08:30, Lad, Prabhakar wrote:

quoted

Hi Guenter,

On Fri, Aug 1, 2025 at 2:52 PM Guenter Roeck [off-list ref] wrote:

quoted

On 8/1/25 04:05, Lad, Prabhakar wrote:

quoted

Hi Wolfram,

Thank you for the review.

On Fri, Aug 1, 2025 at 5:10 AM Wolfram Sang
[off-list ref] wrote:

quoted

On Tue, Jul 29, 2025 at 04:59:13PM +0100, Prabhakar wrote:

quoted

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 ?

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.

Guenter