linux-kernel - Re: [RFC PATCH v2 04/11] time: Add rtc_time_to_tm64() safe version(using time64

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Date:	Thu, 30 Oct 2014 15:07:00 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	"pang.xunlei" <pang.xunlei@...aro.org>
cc:	linux-kernel@...r.kernel.org, rtc-linux@...glegroups.com,
	xen-devel@...ts.xenproject.org,
	John Stultz <john.stultz@...aro.org>,
	Alessandro Zummo <a.zummo@...ertech.it>,
	Stefano Stabellini <stefano.stabellini@...citrix.com>
Subject: Re: [RFC PATCH v2 04/11] time: Add rtc_time_to_tm64() safe version(using
 time64_t)

On Thu, 30 Oct 2014, pang.xunlei wrote:

As you can guess already: $subject sucks.

> +
>  /*
>   * Convert seconds since 01-01-1970 00:00:00 to Gregorian date.
> + * Safe version for 2038 safety.

See previous replies.

>   */
> -void rtc_time_to_tm(unsigned long time, struct rtc_time *tm)
> +void rtc_time_to_tm64(time64_t time, struct rtc_time *tm)

What's tm64? 

This function is converting time64 to rtc_time. So the proper function
name is: rtc_time64_to_tm

Can you see the difference?

>  {
>  	unsigned int month, year;
>  	int days;
>  
> -	days = time / 86400;
> +	days = div_s64(time, 86400);
>  	time -= (unsigned int) days * 86400;
>  
>  	/* day of the week, 1970-01-01 was a Thursday */
> @@ -81,13 +83,23 @@ void rtc_time_to_tm(unsigned long time, struct rtc_time *tm)
>  	tm->tm_mon = month;
>  	tm->tm_mday = days + 1;
>  
> -	tm->tm_hour = time / 3600;
> +	tm->tm_hour = div_s64(time, 3600);

We already adjusted time to a value which is less than 24 * 3600,
i.e. it fits nicely into a long on 32bit. So we can avoid the whole
div_s64 business and simply do:

	unsigned int month, year;
+	unsigned long secs;
	int days;

-	days = time / 86400;
- 	time -= (unsigned int) days * 86400;
+	days = div_s64(time, 86400);
+	secs = time - (unsigned int) days * 86400;

And change the rest which uses time to:

-	tm->tm_hour = time / 3600;
+	tm->tm_hour = secs / 3600;

Sigh.

	tglx
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