[patch 4/5] PTP: add PTP_SYS_OFFSET emulation via cross timestamps infrastructure

[Marcelo Tosatti]

Emulate PTP_SYS_OFFSET by using an arithmetic mean of the
realtime samples from ->getcrosststamp callback.

Note: if the realtime clock changes during cross timestamp sampling,
then the mean will return a value that does not exist. Given the
nature of PTP_SYS_OFFSET users (time synchronization), 
that should not be a problem.

Signed-off-by: Marcelo Tosatti <mtosa...@redhat.com>

---
 drivers/ptp/ptp_chardev.c |   73 ++++++++++++++++++++++++++++++++++++++++++++++
 drivers/ptp/ptp_clock.c   |   10 ++++++
 2 files changed, 83 insertions(+)

v2: drop timekeeper spinlock, move back to drivers/ptp/ptp_chardev.c (Paolo)
    ptp_clock_gettime: support drivers with crosstimestamp but not
    gettime64 callbacks

Index: kvm-ptpdriver/drivers/ptp/ptp_chardev.c
===================================================================
--- kvm-ptpdriver.orig/drivers/ptp/ptp_chardev.c        2017-01-20 
12:48:14.201126341 -0200
+++ kvm-ptpdriver/drivers/ptp/ptp_chardev.c     2017-01-20 12:48:45.270207972 
-0200
@@ -116,6 +116,74 @@
        return 0;
 }
 
+static void mean_sys_realtime(struct system_device_crosststamp *xtstamp,
+                             struct system_device_crosststamp *n_xtstamp,
+                             struct ptp_clock_time *pct)
+{
+       ktime_t sys_realtime, n_sys_realtime;
+       struct timespec ts;
+       u64 ns;
+
+       /* estimate realtime with arithmetic mean of two crosststamp samples */
+       sys_realtime = xtstamp->sys_realtime;
+       n_sys_realtime = n_xtstamp->sys_realtime;
+       ns = ktime_divns(ktime_add(sys_realtime, n_sys_realtime), 2);
+       ts = ktime_to_timespec(ns_to_ktime(ns));
+       pct->sec = ts.tv_sec;
+       pct->nsec = ts.tv_nsec;
+}
+
+static int emulate_ptp_sys_offset(struct ptp_clock_info *info,
+                                 struct ptp_sys_offset *sysoff,
+                                 unsigned long arg)
+{
+       int i, err, len;
+       int n_samples;
+       struct system_device_crosststamp *xtstamps, *xtstamp, *n_xtstamp;
+       struct ptp_clock_time *pct;
+
+       n_samples = sysoff->n_samples + 2;
+
+       len = sizeof(struct system_device_crosststamp);
+       xtstamps = kzalloc(len * n_samples, GFP_KERNEL);
+       if (xtstamps == NULL)
+               return -ENOMEM;
+
+       xtstamp = xtstamps;
+       for (i = 0; i < n_samples; i++) {
+               err = info->getcrosststamp(info, xtstamp);
+               if (err) {
+                       kfree(xtstamps);
+                       return err;
+               }
+               xtstamp++;
+       }
+
+       pct = &sysoff->ts[0];
+       xtstamp = n_xtstamp = xtstamps;
+       n_xtstamp++;
+       for (i = 0; i < sysoff->n_samples; i++) {
+               struct timespec ts;
+
+               mean_sys_realtime(xtstamp, n_xtstamp, pct);
+               pct++;
+
+               ts = ktime_to_timespec(n_xtstamp->device);
+               pct->sec = ts.tv_sec;
+               pct->nsec = ts.tv_nsec;
+               pct++;
+               xtstamp++;
+               n_xtstamp++;
+       }
+
+       mean_sys_realtime(xtstamp, n_xtstamp, pct);
+       kfree(xtstamps);
+       if (copy_to_user((void __user *)arg, sysoff, sizeof(*sysoff)))
+               return -EFAULT;
+
+       return 0;
+}
+
 long ptp_ioctl(struct posix_clock *pc, unsigned int cmd, unsigned long arg)
 {
        struct ptp_clock_caps caps;
@@ -219,6 +287,11 @@
                        err = -EINVAL;
                        break;
                }
+
+               if (!ptp->info->gettime64 && ptp->info->getcrosststamp) {
+                       err = emulate_ptp_sys_offset(ptp->info, sysoff, arg);
+                       break;
+               }
                pct = &sysoff->ts[0];
                for (i = 0; i < sysoff->n_samples; i++) {
                        getnstimeofday64(&ts);
Index: kvm-ptpdriver/drivers/ptp/ptp_clock.c
===================================================================
--- kvm-ptpdriver.orig/drivers/ptp/ptp_clock.c  2017-01-20 12:48:14.201126341 
-0200
+++ kvm-ptpdriver/drivers/ptp/ptp_clock.c       2017-01-20 12:48:45.271207975 
-0200
@@ -118,6 +118,16 @@
        struct timespec64 ts;
        int err;
 
+       if (!ptp->info->gettime64 && ptp->info->getcrosststamp) {
+               struct system_device_crosststamp xtstamp;
+
+               err = ptp->info->getcrosststamp(ptp->info, &xtstamp);
+               if (err)
+                       return err;
+               *tp = ktime_to_timespec(xtstamp.device);
+               return err;
+       }
+
        err = ptp->info->gettime64(ptp->info, &ts);
        if (!err)
                *tp = timespec64_to_timespec(ts);