Hi Guenter, On Fri, Aug 1, 2025 at 2:52 PM Guenter Roeck <linux@xxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxxxx> wrote: > >> > >> On Tue, Jul 29, 2025 at 04:59:13PM +0100, Prabhakar wrote: > >>> From: Lad Prabhakar <prabhakar.mahadev-lad.rj@xxxxxxxxxxxxxx> > >>> > >>> 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. Cheers, Prabhakar
