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([2600:1700:e321:62f0:da43:aeff:fecc:bfd5]) by smtp.gmail.com with ESMTPSA id 98e67ed59e1d1-31f63ee49c7sm8173291a91.21.2025.08.01.14.04.08 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 01 Aug 2025 14:04:09 -0700 (PDT) Sender: Guenter Roeck Message-ID: <6b9338c0-e333-47dd-a3e0-0446b346f008@roeck-us.net> Date: Fri, 1 Aug 2025 14:04:07 -0700 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 7/9] watchdog: rzv2h: Set min_timeout based on max_hw_heartbeat_ms To: "Lad, Prabhakar" Cc: Wolfram Sang , Wim Van Sebroeck , Rob Herring , Krzysztof Kozlowski , Conor Dooley , Philipp Zabel , Geert Uytterhoeven , Magnus Damm , linux-watchdog@vger.kernel.org, devicetree@vger.kernel.org, linux-kernel@vger.kernel.org, linux-renesas-soc@vger.kernel.org, Biju Das , Fabrizio Castro , Lad Prabhakar References: <20250729155915.67758-1-prabhakar.mahadev-lad.rj@bp.renesas.com> <20250729155915.67758-8-prabhakar.mahadev-lad.rj@bp.renesas.com> Content-Language: en-US From: Guenter Roeck Autocrypt: addr=linux@roeck-us.net; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 8/1/25 13:51, Lad, Prabhakar wrote: > Hi Guenter, > > On Fri, Aug 1, 2025 at 7:04 PM Guenter Roeck wrote: >> >> On 8/1/25 08:30, Lad, Prabhakar wrote: >>> Hi Guenter, >>> >>> On Fri, Aug 1, 2025 at 2:52 PM Guenter Roeck 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 >>>>> wrote: >>>>>> >>>>>> On Tue, Jul 29, 2025 at 04:59:13PM +0100, Prabhakar wrote: >>>>>>> From: Lad Prabhakar >>>>>>> >>>>>>> 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. Guenter