time: Introduce auxiliary POSIX clocks

To support auxiliary timekeeping and the related user space interfaces,
it's required to define a clock ID range for them.

Reserve 8 auxiliary clock IDs after the regular timekeeping clock ID space.

This is the maximum number of auxiliary clocks the kernel can support. The actual
number of supported clocks depends obviously on the presence of related devices
and might be constraint by the available VDSO space.

Add the corresponding timekeeper IDs as well.

Signed-off-by: Anna-Maria Behnsen <anna-maria@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: John Stultz <jstultz@google.com>
Link: https://lore.kernel.org/all/20250519083025.905800695@linutronix.de

diff --git a/include/linux/timekeeper_internal.h b/include/linux/timekeeper_internal.h
index bfcecad0e2798fcf972686673b1d99039d489e2f..4201ae818f5704c5a598f074d5b52e263b3a54be 100644 (file)
--- a/include/linux/timekeeper_internal.h
+++ b/include/linux/timekeeper_internal.h
@@ -13,11 +13,17 @@
 
 /**
  * timekeeper_ids - IDs for various time keepers in the kernel
- * @TIMEKEEPER_CORE:   The central core timekeeper managing system time
- * @TIMEKEEPERS_MAX:   The maximum number of timekeepers managed
+ * @TIMEKEEPER_CORE:           The central core timekeeper managing system time
+ * @TIMEKEEPER_AUX_FIRST:      The first AUX timekeeper
+ * @TIMEKEEPER_AUX_LAST:       The last AUX timekeeper
+ * @TIMEKEEPERS_MAX:           The maximum number of timekeepers managed
  */
 enum timekeeper_ids {
        TIMEKEEPER_CORE,
+#ifdef CONFIG_POSIX_AUX_CLOCKS
+       TIMEKEEPER_AUX_FIRST,
+       TIMEKEEPER_AUX_LAST = TIMEKEEPER_AUX_FIRST + MAX_AUX_CLOCKS - 1,
+#endif
        TIMEKEEPERS_MAX,
 };
 

diff --git a/include/uapi/linux/time.h b/include/uapi/linux/time.h
index 4f4b6e48e01c426560483c6c4277228334cbfddf..16ca1ac206fd167476232cc5cb53ae49ffbcdff6 100644 (file)
--- a/include/uapi/linux/time.h
+++ b/include/uapi/linux/time.h
@@ -64,6 +64,17 @@ struct timezone {
 #define CLOCK_TAI                      11
 
 #define MAX_CLOCKS                     16
+
+/*
+ * AUX clock support. AUXiliary clocks are dynamically configured by
+ * enabling a clock ID. These clock can be steered independently of the
+ * core timekeeper. The kernel can support up to 8 auxiliary clocks, but
+ * the actual limit depends on eventual architecture constraints vs. VDSO.
+ */
+#define CLOCK_AUX                      MAX_CLOCKS
+#define MAX_AUX_CLOCKS                 8
+#define CLOCK_AUX_LAST                 (CLOCK_AUX + MAX_AUX_CLOCKS - 1)
+
 #define CLOCKS_MASK                    (CLOCK_REALTIME | CLOCK_MONOTONIC)
 #define CLOCKS_MONO                    CLOCK_MONOTONIC
 

diff --git a/kernel/time/Kconfig b/kernel/time/Kconfig
index b0b97a60aaa6fc21c93005ef393d56afbac7fa5c..7c6a52f7836cef248e0949060b50baa293f446cf 100644 (file)
--- a/kernel/time/Kconfig
+++ b/kernel/time/Kconfig
@@ -82,9 +82,9 @@ config CONTEXT_TRACKING_IDLE
        help
          Tracks idle state on behalf of RCU.
 
-if GENERIC_CLOCKEVENTS
 menu "Timers subsystem"
 
+if GENERIC_CLOCKEVENTS
 # Core internal switch. Selected by NO_HZ_COMMON / HIGH_RES_TIMERS. This is
 # only related to the tick functionality. Oneshot clockevent devices
 # are supported independent of this.
@@ -208,6 +208,17 @@ config CLOCKSOURCE_WATCHDOG_MAX_SKEW_US
          interval and NTP's maximum frequency drift of 500 parts
          per million.  If the clocksource is good enough for NTP,
          it is good enough for the clocksource watchdog!
+endif
+
+config POSIX_AUX_CLOCKS
+       bool "Enable auxiliary POSIX clocks"
+       depends on POSIX_TIMERS
+       help
+         Auxiliary POSIX clocks are clocks which can be steered
+         independently of the core timekeeper, which controls the
+         MONOTONIC, REALTIME, BOOTTIME and TAI clocks.  They are useful to
+         provide e.g. lockless time accessors to independent PTP clocks
+         and other clock domains, which are not correlated to the TAI/NTP
+         notion of time.
 
 endmenu
-endif