linux-kernel - [PATCH] watchdog: rti_wdt: Fix calculation and evaluation of preset heartbeat

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Message-ID: <6a4b54ac-9588-e172-c4c7-b91d524a851e@siemens.com>
Date:   Mon, 21 Feb 2022 17:22:38 +0100
From:   Jan Kiszka <jan.kiszka@...mens.com>
To:     Hari Nagalla <hnagalla@...com>, Nishanth Menon <nm@...com>,
        "Wim Van Sebroeck" <wim@...ux-watchdog.org>,
        Guenter Roeck <linux@...ck-us.net>
CC:     <linux-watchdog@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: [PATCH] watchdog: rti_wdt: Fix calculation and evaluation of preset
 heartbeat

From: Jan Kiszka <jan.kiszka@...mens.com>

This ensures that the same value is read back as was eventually
programmed when using seconds as accuracy. Even then, comparing the more
precise heartbeat_ms against heartbeat in seconds will almost never
provide a match and will needlessly raise a warning. Fix by comparing
apples to apples.

Tested in combination with U-Boot as watchdog starter.

Signed-off-by: Jan Kiszka <jan.kiszka@...mens.com>
---

To fully work, it also needs a U-Boot patch to add the same 10% margin 
to the clock frequency:

https://patchwork.ozlabs.org/project/uboot/patch/9b3603a3-1bf4-eb57-c378-20d5c2accc34@siemens.com/

 drivers/watchdog/rti_wdt.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/drivers/watchdog/rti_wdt.c b/drivers/watchdog/rti_wdt.c
index 117bc2a8eb0a..3ecd14e180fb 100644
--- a/drivers/watchdog/rti_wdt.c
+++ b/drivers/watchdog/rti_wdt.c
@@ -252,6 +252,7 @@ static int rti_wdt_probe(struct platform_device *pdev)
 	}
 
 	if (readl(wdt->base + RTIDWDCTRL) == WDENABLE_KEY) {
+		int preset_heartbeat;
 		u32 time_left_ms;
 		u64 heartbeat_ms;
 		u32 wsize;
@@ -262,11 +263,12 @@ static int rti_wdt_probe(struct platform_device *pdev)
 		heartbeat_ms <<= WDT_PRELOAD_SHIFT;
 		heartbeat_ms *= 1000;
 		do_div(heartbeat_ms, wdt->freq);
-		if (heartbeat_ms != heartbeat * 1000)
+		preset_heartbeat = heartbeat_ms + 500;
+		preset_heartbeat /= 1000;
+		if (preset_heartbeat != heartbeat)
 			dev_warn(dev, "watchdog already running, ignoring heartbeat config!\n");
 
-		heartbeat = heartbeat_ms;
-		heartbeat /= 1000;
+		heartbeat = preset_heartbeat;
 
 		wsize = readl(wdt->base + RTIWWDSIZECTRL);
 		ret = rti_wdt_setup_hw_hb(wdd, wsize);
-- 
2.34.1