powerpc: Fix problem with time going backwards

[linux-2.6-omap-h63xx.git] / arch / powerpc / kernel / time.c

diff --git a/arch/powerpc/kernel/time.c b/arch/powerpc/kernel/time.c
index 2a7ddc5793797ecb9d2146e415dee6740e815d49..86f7e3d154d8a11065e16064d4dc083fa99d115c 100644 (file)
--- a/arch/powerpc/kernel/time.c
+++ b/arch/powerpc/kernel/time.c
@@ -283,9 +283,9 @@ static inline void update_gtod(u64 new_tb_stamp, u64 new_stamp_xsec,
         * the two values of tb_update_count match and are even then the
         * tb_to_xs and stamp_xsec values are consistent.  If not, then it
         * loops back and reads them again until this criteria is met.
+        * We expect the caller to have done the first increment of
+        * vdso_data->tb_update_count already.
         */
-       ++(vdso_data->tb_update_count);
-       smp_wmb();
        vdso_data->tb_orig_stamp = new_tb_stamp;
        vdso_data->stamp_xsec = new_stamp_xsec;
        vdso_data->tb_to_xs = new_tb_to_xs;
@@ -310,20 +310,15 @@ static __inline__ void timer_recalc_offset(u64 cur_tb)
        unsigned long offset;
        u64 new_stamp_xsec;
        u64 tlen, t2x;
+       u64 tb, xsec_old, xsec_new;
+       struct gettimeofday_vars *varp;
 
        if (__USE_RTC())
                return;
        tlen = current_tick_length();
        offset = cur_tb - do_gtod.varp->tb_orig_stamp;
-       if (tlen == last_tick_len && offset < 0x80000000u) {
-               /* check that we're still in sync; if not, resync */
-               struct timeval tv;
-               __do_gettimeofday(&tv, cur_tb);
-               if (tv.tv_sec <= xtime.tv_sec &&
-                   (tv.tv_sec < xtime.tv_sec ||
-                    tv.tv_usec * 1000 <= xtime.tv_nsec))
-                       return;
-       }
+       if (tlen == last_tick_len && offset < 0x80000000u)
+               return;
        if (tlen != last_tick_len) {
                t2x = mulhdu(tlen << TICKLEN_SHIFT, ticklen_to_xs);
                last_tick_len = tlen;
@@ -332,6 +327,21 @@ static __inline__ void timer_recalc_offset(u64 cur_tb)
        new_stamp_xsec = (u64) xtime.tv_nsec * XSEC_PER_SEC;
        do_div(new_stamp_xsec, 1000000000);
        new_stamp_xsec += (u64) xtime.tv_sec * XSEC_PER_SEC;
+
+       ++vdso_data->tb_update_count;
+       smp_mb();
+
+       /*
+        * Make sure time doesn't go backwards for userspace gettimeofday.
+        */
+       tb = get_tb();
+       varp = do_gtod.varp;
+       xsec_old = mulhdu(tb - varp->tb_orig_stamp, varp->tb_to_xs)
+               + varp->stamp_xsec;
+       xsec_new = mulhdu(tb - cur_tb, t2x) + new_stamp_xsec;
+       if (xsec_new < xsec_old)
+               new_stamp_xsec += xsec_old - xsec_new;
+
        update_gtod(cur_tb, new_stamp_xsec, t2x);
 }
 
@@ -564,6 +574,10 @@ int do_settimeofday(struct timespec *tv)
        }
 #endif
 
+       /* Make userspace gettimeofday spin until we're done. */
+       ++vdso_data->tb_update_count;
+       smp_mb();
+
        /*
         * Subtract off the number of nanoseconds since the
         * beginning of the last tick.
@@ -724,10 +738,16 @@ void __init time_init(void)
         * It is computed as:
         * ticklen_to_xs = 2^N / (tb_ticks_per_jiffy * 1e9)
         * where N = 64 + 20 - TICKLEN_SCALE - TICKLEN_SHIFT
-        * so as to give the result as a 0.64 fixed-point fraction.
+        * which turns out to be N = 51 - SHIFT_HZ.
+        * This gives the result as a 0.64 fixed-point fraction.
+        * That value is reduced by an offset amounting to 1 xsec per
+        * 2^31 timebase ticks to avoid problems with time going backwards
+        * by 1 xsec when we do timer_recalc_offset due to losing the
+        * fractional xsec.  That offset is equal to ppc_tb_freq/2^51
+        * since there are 2^20 xsec in a second.
         */
-       div128_by_32(1ULL << (64 + 20 - TICKLEN_SCALE - TICKLEN_SHIFT), 0,
-                    tb_ticks_per_jiffy, &res);
+       div128_by_32((1ULL << 51) - ppc_tb_freq, 0,
+                    tb_ticks_per_jiffy << SHIFT_HZ, &res);
        div128_by_32(res.result_high, res.result_low, NSEC_PER_SEC, &res);
        ticklen_to_xs = res.result_low;