s390/time: convert tod_clock_base to union

Convert tod_clock_base to union tod_clock. This simplifies quite a bit
of code and also fixes a bug in read_persistent_clock64();

void read_persistent_clock64(struct timespec64 *ts)
{
        __u64 delta;

        delta = initial_leap_seconds + TOD_UNIX_EPOCH;
        get_tod_clock_ext(clk);
        *(__u64 *) &clk[1] -= delta;
        if (*(__u64 *) &clk[1] > delta)
                clk[0]--;
        ext_to_timespec64(clk, ts);
}

Assume &clk[1] == 3 and delta == 2; then after the substraction the if
condition becomes true and the epoch part of the clock is decremented
by one because of an assumed overflow, even though there is none.

Fix this by using 128 bit arithmetics and let the compiler do the
right thing:

void read_persistent_clock64(struct timespec64 *ts)
{
        union tod_clock clk;
        u64 delta;

        delta = initial_leap_seconds + TOD_UNIX_EPOCH;
        store_tod_clock_ext(&clk);
        clk.eitod -= delta;
        ext_to_timespec64(&clk, ts);
}

Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

1 files changed, 3 insertions, 3 deletions

diff --git a/arch/s390/kernel/early.c b/arch/s390/kernel/early.c
index 8f046f985d8e..a361d2e70025 100644
--- a/arch/s390/kernel/early.c
+++ b/arch/s390/kernel/early.c
@@ -43,8 +43,8 @@ static void __init reset_tod_clock(void)
 	if (set_tod_clock(TOD_UNIX_EPOCH) || store_tod_clock_ext_cc(&clk))
 		disabled_wait();
 
-	memset(tod_clock_base, 0, 16);
-	*(__u64 *) &tod_clock_base[1] = TOD_UNIX_EPOCH;
+	memset(&tod_clock_base, 0, sizeof(tod_clock_base));
+	tod_clock_base.tod = TOD_UNIX_EPOCH;
 	S390_lowcore.last_update_clock = TOD_UNIX_EPOCH;
 }
 
@@ -230,7 +230,7 @@ static __init void detect_machine_facilities(void)
 	}
 	if (test_facility(133))
 		S390_lowcore.machine_flags |= MACHINE_FLAG_GS;
-	if (test_facility(139) && (tod_clock_base[1] & 0x80)) {
+	if (test_facility(139) && (tod_clock_base.tod >> 63)) {
 		/* Enabled signed clock comparator comparisons */
 		S390_lowcore.machine_flags |= MACHINE_FLAG_SCC;
 		clock_comparator_max = -1ULL >> 1;