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 <a.zummo@xxxxxxxxxxxx>
Cc: Alexandre Belloni <alexandre.belloni@xxxxxxxxxxxxxxxxxx>
Cc: rtc-linux@xxxxxxxxxxxxxxxx
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>
---
v2:
- optimize compute_wday() to prevent adding latency to the common case to
accommodate the exceptional case (i.e., RTC date << present date)
- move year validation to convert_from_efi_time() and check the upper bound
as well (<= 9999)
drivers/rtc/rtc-efi.c | 39 ++++++++++++++-------------------------
1 file changed, 14 insertions(+), 25 deletions(-)
diff --git a/drivers/rtc/rtc-efi.c b/drivers/rtc/rtc-efi.c
index cb989cd00b14..6e917e2f5e86 100644
--- a/drivers/rtc/rtc-efi.c
+++ b/drivers/rtc/rtc-efi.c
@@ -24,10 +24,6 @@
#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]
@@ -38,31 +34,24 @@ compute_yday(efi_time_t *eft)
/* 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
@@ -103,16 +92,16 @@ convert_from_efi_time(efi_time_t *eft, struct rtc_time *wtime)
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:
--
1.9.1
--
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