linux-kernel - Re: [PATCH v2 1/2] clocksource/drivers/timer-tegra186: add WDIOC

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Date: Sat, 24 Feb 2024 07:51:26 +0800
From: Pohsun Su <pohsuns@...dia.com>
To: <thierry.reding@...il.com>, <daniel.lezcano@...aro.org>,
	<tglx@...utronix.de>, <jonathanh@...dia.com>
CC: <linux-kernel@...r.kernel.org>, <linux-tegra@...r.kernel.org>,
	<sumitg@...dia.com>, Pohsun Su <pohsuns@...dia.com>
Subject: Re: [PATCH v2 1/2] clocksource/drivers/timer-tegra186: add WDIOC_GETTIMELEFT support

Hi Thierry,

>> +static unsigned int tegra186_wdt_get_timeleft(struct watchdog_device *wdd)
>> +{
>> +     struct tegra186_wdt *wdt = to_tegra186_wdt(wdd);
>> +     u32 timeleft;
>> +     u32 expiration;
>> +
>> +     if (!watchdog_active(&wdt->base)) {
>> +             /* return zero if the watchdog timer is not activated. */
>> +             return 0;
>> +     }
>> +
>> +     /*
>> +      * System power-on reset occurs on the fifth expiration of the watchdog timer and so
>
>Is "system power-on reset" really what this is called? Power-on reset
>sounds like something that only happens after you power the device on,
>not something that can be triggered by the watchdog.

I will change it from "system power-on reset" to "System POR(Power On Reset)" in next patch.
AFAIK Power On Reset is used for decribing resetting circuits and initialing whatever it needs
when received a POR signal after powered up. This term should also be applicable for
hardware watchdog reset since the system is already powered up at that moment and 

>> +      * when the watchdog timer is configured, the actual value programmed into the counter
>> +      * is 1/5 of the timeout value. Once the counter reaches 0, expiration count will be
>> +      * increased by 1 and the down counter restarts.
>> +      * Hence to get the time left before system reset we must combine 2 parts:
>> +      * 1. value of the current down counter
>> +      * 2. (number of counter expirations remaining) * (timeout/5)
>> +      */
>
>Can you wrap this comment so that it fits within 80 columns? It's fine
>to occasionally go beyond that limit if there's a good reason for it,
>but this comment doesn't seem to fall into that category.

Sorry, I missed to check the line length before submit,
will wrap comments in next patch.

>> +
>> +     /* Get the current number of counter expirations. Should be a value between 0 and 4. */
>> +     expiration = FIELD_GET(WDTSR_CURRENT_EXPIRATION_COUNT, readl_relaxed(wdt->regs + WDTSR));
>> +
>> +     /* Convert the current counter value to seconds, rounding up to the nearest second. */
>> +     timeleft = FIELD_GET(TMRSR_PCV, readl_relaxed(wdt->tmr->regs + TMRSR));
>> +     timeleft = (timeleft + USEC_PER_SEC / 2) / USEC_PER_SEC;
>
>Same for these. Maybe make an extra variable to store the register value
>in to get rid of some of that extra horizontal space.

Thanks for the suggestion, will do.

>> +
>> +     /*
>> +      * Calculate the time remaining by adding the time for the counter value
>> +      * to the time of the counter expirations that remain.
>> +      */
>> +     timeleft += wdt->base.timeout * (4 - expiration) / 5;
>
>This doesn't quite match what the comment above says. Shouldn't this be:
>
>        timeleft += (wdt->base.timeout / 5) * (4 - expiration);
>
>instead?

Here I made a transposition on purpose just for keeping the precision due to the integer division.
e.g. If 'divided by 5' goes first, the equation goes to 0 when wdt->base.timeout is less than 5
 no matter which timer expiration it is.

But the number will be still inaccurate because I made a mistake that
the number should be calculated in microseconds all the time 
and be converted to a nearest second only in the last step.

A new patch has been made and I will submit it after verifications.
Thank you for your time on reviewing this!

Best,
--
Pohsun