rtc: efi: use correct EFI 'epoch'

The rtc-efi driver declares that the EFI 'epoch' is 1/1/1998, but
the UEFI spec does not define it at all. It does define a range of
[1900, 9999] for the 'Year' member of the EFI_TIME struct, so let's
use 1900 as the minimum year and not 1998.
Also, move the validation of the year to the convert_from_efi_time()
routine where all other EFI_TIME fields are validated as well.

This prevents rtc_read_time() failures when the RTC that backs the
EFI time services is set to a date before 1998, e.g., when it has
lost power.

This also optimizes the compute_wday() routine, by replacing the for
loop with a simple arithmetic expression, and by reusing the yearday
value that we need to compute anyway when populating the
rtc_time::tm_yday field.

Cc: Alessandro Zummo <[e-mail]>
Cc: Alexandre Belloni <[e-mail]>
Cc: [e-mail]
Signed-off-by: Ard Biesheuvel <[e-mail]>
Signed-off-by: Alexandre Belloni <[e-mail]>

drivers/rtc/rtc-efi.c

···
 #include <linux/efi.h>
 
 #define EFI_ISDST (EFI_TIME_ADJUST_DAYLIGHT|EFI_TIME_IN_DAYLIGHT)
-/*
- * EFI Epoch is 1/1/1998
- */
-#define EFI_RTC_EPOCH		1998
 
 /*
  * returns day of the year [0-365]
···
 	/* efi_time_t.month is in the [1-12] so, we need -1 */
 	return rtc_year_days(eft->day, eft->month - 1, eft->year);
 }
+
 /*
  * returns day of the week [0-6] 0=Sunday
- *
- * Don't try to provide a year that's before 1998, please !
  */
 static int
-compute_wday(efi_time_t *eft)
+compute_wday(efi_time_t *eft, int yday)
 {
-	int y;
-	int ndays = 0;
-
-	if (eft->year < EFI_RTC_EPOCH) {
-		pr_err("EFI year < " __stringify(EFI_RTC_EPOCH) ", invalid date\n");
-		return -1;
-	}
-
-	for (y = EFI_RTC_EPOCH; y < eft->year; y++)
-		ndays += 365 + (is_leap_year(y) ? 1 : 0);
-
-	ndays += compute_yday(eft);
+	int ndays = eft->year * (365 % 7)
+		    + (eft->year - 1) / 4
+		    - (eft->year - 1) / 100
+		    + (eft->year - 1) / 400
+		    + yday;
 
 	/*
-	 * 4=1/1/1998 was a Thursday
+	 * 1/1/0000 may or may not have been a Sunday (if it ever existed at
+	 * all) but assuming it was makes this calculation work correctly.
 	 */
-	return (ndays + 4) % 7;
+	return ndays % 7;
 }
 
 static void
···
 	if (!eft->month || eft->month > 12)
 		return false;
 	wtime->tm_mon  = eft->month - 1;
-	wtime->tm_year = eft->year - 1900;
 
-	/* day of the week [0-6], Sunday=0 */
-	wtime->tm_wday = compute_wday(eft);
-	if (wtime->tm_wday < 0)
+	if (eft->year < 1900 || eft->year > 9999)
 		return false;
+	wtime->tm_year = eft->year - 1900;
 
 	/* day in the year [1-365]*/
 	wtime->tm_yday = compute_yday(eft);
 
+	/* day of the week [0-6], Sunday=0 */
+	wtime->tm_wday = compute_wday(eft, wtime->tm_yday);
 
 	switch (eft->daylight & EFI_ISDST) {
 	case EFI_ISDST: