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Message-ID: <18adfcd0-e955-4c3f-a68a-6a2f75ebd24d@roeck-us.net>
Date: Mon, 4 Aug 2025 21:47:12 -0700
From: Guenter Roeck <linux@...ck-us.net>
To: sw617.shin@...sung.com, 'Sam Protsenko' <semen.protsenko@...aro.org>
Cc: krzk@...nel.org, alim.akhtar@...sung.com, wim@...ux-watchdog.org,
khwan.seo@...sung.com, dongil01.park@...sung.com,
linux-arm-kernel@...ts.infradead.org, linux-samsung-soc@...r.kernel.org,
linux-watchdog@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 2/4] watchdog: s3c2410_wdt: Fix max_timeout being
calculated larger
On 8/4/25 21:22, sw617.shin@...sung.com wrote:
> On Saturday, August 2, 2025 at 1:12 PM Sam Protsenko <semen.protsenko@...aro.org> wrote:
>
>> How about something like this instead?
>>
>> 8<--------------------------------------------------------------------->8
>> static inline unsigned int s3c2410wdt_max_timeout(unsigned long freq) {
>> const u64 div_max = (S3C2410_WTCON_PRESCALE_MAX + 1) *
>> S3C2410_WTCON_MAXDIV; /* 32768 */
>> const u64 n_max = S3C2410_WTCNT_MAXCNT * div_max;
>> u64 t_max = n_max / freq;
>>
>> if (t_max > UINT_MAX)
>> t_max = UINT_MAX;
>>
>> return (unsigned int)t_max;
>> }
>> 8<--------------------------------------------------------------------->8
>>
>> This implementation's result:
>> - is never greater than real timeout, as it loses the decimal part after
>> integer division in t_max
>> - much closer to the real timeout value, as it benefits from very big
>> n_max in the numerator (this is the main trick here)
>> - prepared for using 32-bit max counter value in your next patch, as it
>> uses u64 type for calculations
>>
>> For example, at the clock frequency of 33 kHz:
>> - real timeout is: 65074.269 sec
>> - old function returns: 65535 sec
>> - your function returns: 32767 sec
>> - the suggested function returns: 65074 sec
>
> Thank you for your feedback.
> I'll make the code changes as follows in the next patch set:
>
> static inline unsigned int s3c2410wdt_max_timeout(struct s3c2410_wdt *wdt)
> {
> const unsigned long freq = s3c2410wdt_get_freq(wdt);
> + const u64 div_max = (S3C2410_WTCON_PRESCALE_MAX + 1) *
> + S3C2410_WTCON_MAXDIV;
> + const u64 n_max = S3C2410_WTCNT_MAXCNT * div_max;
Not sure if splitting this expression adds any value. Why not just the following ?
const u64 n_max = (u64)(S3C2410_WTCON_PRESCALE_MAX + 1) *
S3C2410_WTCON_MAXDIV * S3C2410_WTCNT_MAXCNT;
Or just use a define ?
> + u64 t_max = n_max / freq;
>
Make sure this compiles on 32-bit builds.
> - return S3C2410_WTCNT_MAXCNT / (freq / (S3C2410_WTCON_PRESCALE_MAX + 1)
> - / S3C2410_WTCON_MAXDIV);
> + if (t_max > UINT_MAX)
> + t_max = UINT_MAX;
> +
> + return (unsigned int)t_max;
I am quite sure that this typecast is unnecessary.
Guenter
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