From 8006c24595cab106bcb9da12d35e32e14ff492df Mon Sep 17 00:00:00 2001 From: "Christopher S. Hall" Date: Mon, 22 Feb 2016 03:15:22 -0800 Subject: [PATCH] time: Add driver cross timestamp interface for higher precision time synchronization ACKNOWLEDGMENT: cross timestamp code was developed by Thomas Gleixner . It has changed considerably and any mistakes are mine. The precision with which events on multiple networked systems can be synchronized using, as an example, PTP (IEEE 1588, 802.1AS) is limited by the precision of the cross timestamps between the system clock and the device (timestamp) clock. Precision here is the degree of simultaneity when capturing the cross timestamp. Currently the PTP cross timestamp is captured in software using the PTP device driver ioctl PTP_SYS_OFFSET. Reads of the device clock are interleaved with reads of the realtime clock. At best, the precision of this cross timestamp is on the order of several microseconds due to software latencies. Sub-microsecond precision is required for industrial control and some media applications. To achieve this level of precision hardware supported cross timestamping is needed. The function get_device_system_crosstimestamp() allows device drivers to return a cross timestamp with system time properly scaled to nanoseconds. The realtime value is needed to discipline that clock using PTP and the monotonic raw value is used for applications that don't require a "real" time, but need an unadjusted clock time. The get_device_system_crosstimestamp() code calls back into the driver to ensure that the system counter is within the current timekeeping update interval. Modern Intel hardware provides an Always Running Timer (ART) which is exactly related to TSC through a known frequency ratio. The ART is routed to devices on the system and is used to precisely and simultaneously capture the device clock with the ART. Cc: Prarit Bhargava Cc: Richard Cochran Cc: Thomas Gleixner Cc: Ingo Molnar Cc: Andy Lutomirski Cc: kevin.b.stanton@intel.com Cc: kevin.j.clarke@intel.com Cc: hpa@zytor.com Cc: jeffrey.t.kirsher@intel.com Cc: netdev@vger.kernel.org Reviewed-by: Thomas Gleixner Signed-off-by: Christopher S. Hall [jstultz: Reworked to remove extra structures and simplify calling] Signed-off-by: John Stultz --- include/linux/timekeeping.h | 35 +++++++++++++++++++++++ kernel/time/timekeeping.c | 56 +++++++++++++++++++++++++++++++++++++ 2 files changed, 91 insertions(+) diff --git a/include/linux/timekeeping.h b/include/linux/timekeeping.h index 7817591af46f..4a2ca65fc778 100644 --- a/include/linux/timekeeping.h +++ b/include/linux/timekeeping.h @@ -279,6 +279,41 @@ struct system_time_snapshot { ktime_t raw; }; +/* + * struct system_device_crosststamp - system/device cross-timestamp + * (syncronized capture) + * @device: Device time + * @sys_realtime: Realtime simultaneous with device time + * @sys_monoraw: Monotonic raw simultaneous with device time + */ +struct system_device_crosststamp { + ktime_t device; + ktime_t sys_realtime; + ktime_t sys_monoraw; +}; + +/* + * struct system_counterval_t - system counter value with the pointer to the + * corresponding clocksource + * @cycles: System counter value + * @cs: Clocksource corresponding to system counter value. Used by + * timekeeping code to verify comparibility of two cycle values + */ +struct system_counterval_t { + cycle_t cycles; + struct clocksource *cs; +}; + +/* + * Get cross timestamp between system clock and device clock + */ +extern int get_device_system_crosststamp( + int (*get_time_fn)(ktime_t *device_time, + struct system_counterval_t *system_counterval, + void *ctx), + void *ctx, + struct system_device_crosststamp *xtstamp); + /* * Simultaneously snapshot realtime and monotonic raw clocks */ diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c index af19a49d5223..dba595cdb200 100644 --- a/kernel/time/timekeeping.c +++ b/kernel/time/timekeeping.c @@ -907,6 +907,62 @@ void ktime_get_snapshot(struct system_time_snapshot *systime_snapshot) } EXPORT_SYMBOL_GPL(ktime_get_snapshot); +/** + * get_device_system_crosststamp - Synchronously capture system/device timestamp + * @sync_devicetime: Callback to get simultaneous device time and + * system counter from the device driver + * @xtstamp: Receives simultaneously captured system and device time + * + * Reads a timestamp from a device and correlates it to system time + */ +int get_device_system_crosststamp(int (*get_time_fn) + (ktime_t *device_time, + struct system_counterval_t *sys_counterval, + void *ctx), + void *ctx, + struct system_device_crosststamp *xtstamp) +{ + struct system_counterval_t system_counterval; + struct timekeeper *tk = &tk_core.timekeeper; + ktime_t base_real, base_raw; + s64 nsec_real, nsec_raw; + unsigned long seq; + int ret; + + do { + seq = read_seqcount_begin(&tk_core.seq); + /* + * Try to synchronously capture device time and a system + * counter value calling back into the device driver + */ + ret = get_time_fn(&xtstamp->device, &system_counterval, ctx); + if (ret) + return ret; + + /* + * Verify that the clocksource associated with the captured + * system counter value is the same as the currently installed + * timekeeper clocksource + */ + if (tk->tkr_mono.clock != system_counterval.cs) + return -ENODEV; + + base_real = ktime_add(tk->tkr_mono.base, + tk_core.timekeeper.offs_real); + base_raw = tk->tkr_raw.base; + + nsec_real = timekeeping_cycles_to_ns(&tk->tkr_mono, + system_counterval.cycles); + nsec_raw = timekeeping_cycles_to_ns(&tk->tkr_raw, + system_counterval.cycles); + } while (read_seqcount_retry(&tk_core.seq, seq)); + + xtstamp->sys_realtime = ktime_add_ns(base_real, nsec_real); + xtstamp->sys_monoraw = ktime_add_ns(base_raw, nsec_raw); + return 0; +} +EXPORT_SYMBOL_GPL(get_device_system_crosststamp); + /** * do_gettimeofday - Returns the time of day in a timeval * @tv: pointer to the timeval to be set -- 2.39.5