rtc: Allow rtc drivers to specify the tv_nsec value for ntp

ntp is currently hardwired to try and call the rtc set when wall clock
tv_nsec is 0.5 seconds. This historical behaviour works well with certain
PC RTCs, but is not universal to all rtc hardware.

Change how this works by introducing the driver specific concept of
set_offset_nsec, the delay between current wall clock time and the target
time to set (with a 0 tv_nsecs).

For x86-style CMOS set_offset_nsec should be -0.5 s which causes the last
second to be written 0.5 s after it has started.

For compat with the old rtc_set_ntp_time, the value is defaulted to
+ 0.5 s, which causes the next second to be written 0.5s before it starts,
as things were before this patch.

Testing shows many non-x86 RTCs would like set_offset_nsec ~= 0,
so ultimately each RTC driver should set the set_offset_nsec according
to its needs, and non x86 architectures should stop using
update_persistent_clock64 in order to access this feature.
Future patches will revise the drivers as needed.

Since CMOS and RTC now have very different handling they are split
into two dedicated code paths, sharing the support code, and ifdefs
are replaced with IS_ENABLED.

Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Miroslav Lichvar <mlichvar@redhat.com>
Cc: Richard Cochran <richardcochran@gmail.com>
Cc: Prarit Bhargava <prarit@redhat.com>
Cc: Stephen Boyd <stephen.boyd@linaro.org>
Signed-off-by: Jason Gunthorpe <jgunthorpe@obsidianresearch.com>
Signed-off-by: John Stultz <john.stultz@linaro.org>

1 files changed, 42 insertions, 1 deletions

diff --git a/include/linux/rtc.h b/include/linux/rtc.h
index e6d0f9c1cafd..5b13fa029fd6 100644
--- a/include/linux/rtc.h
+++ b/include/linux/rtc.h
@@ -135,6 +135,14 @@ struct rtc_device {
         /* Some hardware can't support UIE mode */
         int uie_unsupported;
 
+        /* Number of nsec it takes to set the RTC clock. This influences when
+         * the set ops are called. An offset:
+         *   - of 0.5 s will call RTC set for wall clock time 10.0 s at 9.5 s
+         *   - of 1.5 s will call RTC set for wall clock time 10.0 s at 8.5 s
+         *   - of -0.5 s will call RTC set for wall clock time 10.0 s at 10.5 s
+         */
+        long set_offset_nsec;
+
         bool registered;
 
         struct nvmem_config *nvmem_config;
@@ -172,7 +180,7 @@ extern void devm_rtc_device_unregister(struct device *dev,
 
 extern int rtc_read_time(struct rtc_device *rtc, struct rtc_time *tm);
 extern int rtc_set_time(struct rtc_device *rtc, struct rtc_time *tm);
-extern int rtc_set_ntp_time(struct timespec64 now);
+extern int rtc_set_ntp_time(struct timespec64 now, unsigned long *target_nsec);
 int __rtc_read_alarm(struct rtc_device *rtc, struct rtc_wkalrm *alarm);
 extern int rtc_read_alarm(struct rtc_device *rtc,
                         struct rtc_wkalrm *alrm);
@@ -221,6 +229,39 @@ static inline bool is_leap_year(unsigned int year)
         return (!(year % 4) && (year % 100)) || !(year % 400);
 }
 
+/* Determine if we can call to driver to set the time. Drivers can only be
+ * called to set a second aligned time value, and the field set_offset_nsec
+ * specifies how far away from the second aligned time to call the driver.
+ *
+ * This also computes 'to_set' which is the time we are trying to set, and has
+ * a zero in tv_nsecs, such that:
+ *    to_set - set_delay_nsec == now +/- FUZZ
+ *
+ */
+static inline bool rtc_tv_nsec_ok(s64 set_offset_nsec,
+                                  struct timespec64 *to_set,
+                                  const struct timespec64 *now)
+{
+        /* Allowed error in tv_nsec, arbitarily set to 5 jiffies in ns. */
+        const unsigned long TIME_SET_NSEC_FUZZ = TICK_NSEC * 5;
+        struct timespec64 delay = {.tv_sec = 0,
+                                   .tv_nsec = set_offset_nsec};
+
+        *to_set = timespec64_add(*now, delay);
+
+        if (to_set->tv_nsec < TIME_SET_NSEC_FUZZ) {
+                to_set->tv_nsec = 0;
+                return true;
+        }
+
+        if (to_set->tv_nsec > NSEC_PER_SEC - TIME_SET_NSEC_FUZZ) {
+                to_set->tv_sec++;
+                to_set->tv_nsec = 0;
+                return true;
+        }
+        return false;
+}
+
 #define rtc_register_device(device) \
         __rtc_register_device(THIS_MODULE, device)