linux-kernel - [PATCH] timekeeping: Remove unused ktime_get_fast

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Message-ID: <20250112160132.450209-1-linux@treblig.org>
Date: Sun, 12 Jan 2025 16:01:32 +0000
From: linux@...blig.org
To: jstultz@...gle.com,
	tglx@...utronix.de,
	sboyd@...nel.org
Cc: linux-kernel@...r.kernel.org,
	"Dr. David Alan Gilbert" <linux@...blig.org>
Subject: [PATCH] timekeeping: Remove unused ktime_get_fast_timestamps

From: "Dr. David Alan Gilbert" <linux@...blig.org>

ktime_get_fast_timestamps() was added in 2020 by
commit e2d977c9f1ab ("timekeeping: Provide multi-timestamp accessor to
NMI safe timekeeper")

but has remained unused.

Remove it.

(Perhaps it's worth just reverting the entire commit, the only bit
left is the extra parameter on __ktime_get_real_fast which is now
always NULL).

Signed-off-by: Dr. David Alan Gilbert <linux@...blig.org>
---
 include/linux/timekeeping.h | 15 -----------
 kernel/time/timekeeping.c   | 54 -------------------------------------
 2 files changed, 69 deletions(-)

diff --git a/include/linux/timekeeping.h b/include/linux/timekeeping.h
index 0e035f675efe..542773650200 100644
--- a/include/linux/timekeeping.h
+++ b/include/linux/timekeeping.h
@@ -263,18 +263,6 @@ extern bool timekeeping_rtc_skipresume(void);
 
 extern void timekeeping_inject_sleeptime64(const struct timespec64 *delta);
 
-/**
- * struct ktime_timestamps - Simultaneous mono/boot/real timestamps
- * @mono:	Monotonic timestamp
- * @boot:	Boottime timestamp
- * @real:	Realtime timestamp
- */
-struct ktime_timestamps {
-	u64		mono;
-	u64		boot;
-	u64		real;
-};
-
 /**
  * struct system_time_snapshot - simultaneous raw/real time capture with
  *				 counter value
@@ -345,9 +333,6 @@ extern int get_device_system_crosststamp(
  */
 extern void ktime_get_snapshot(struct system_time_snapshot *systime_snapshot);
 
-/* NMI safe mono/boot/realtime timestamps */
-extern void ktime_get_fast_timestamps(struct ktime_timestamps *snap);
-
 /*
  * Persistent clock related interfaces
  */
diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index 3d128825d343..2f9de089bc0d 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -515,60 +515,6 @@ u64 ktime_get_real_fast_ns(void)
 }
 EXPORT_SYMBOL_GPL(ktime_get_real_fast_ns);
 
-/**
- * ktime_get_fast_timestamps: - NMI safe timestamps
- * @snapshot:	Pointer to timestamp storage
- *
- * Stores clock monotonic, boottime and realtime timestamps.
- *
- * Boot time is a racy access on 32bit systems if the sleep time injection
- * happens late during resume and not in timekeeping_resume(). That could
- * be avoided by expanding struct tk_read_base with boot offset for 32bit
- * and adding more overhead to the update. As this is a hard to observe
- * once per resume event which can be filtered with reasonable effort using
- * the accurate mono/real timestamps, it's probably not worth the trouble.
- *
- * Aside of that it might be possible on 32 and 64 bit to observe the
- * following when the sleep time injection happens late:
- *
- * CPU 0				CPU 1
- * timekeeping_resume()
- * ktime_get_fast_timestamps()
- *	mono, real = __ktime_get_real_fast()
- *					inject_sleep_time()
- *					   update boot offset
- *	boot = mono + bootoffset;
- *
- * That means that boot time already has the sleep time adjustment, but
- * real time does not. On the next readout both are in sync again.
- *
- * Preventing this for 64bit is not really feasible without destroying the
- * careful cache layout of the timekeeper because the sequence count and
- * struct tk_read_base would then need two cache lines instead of one.
- *
- * Access to the time keeper clock source is disabled across the innermost
- * steps of suspend/resume. The accessors still work, but the timestamps
- * are frozen until time keeping is resumed which happens very early.
- *
- * For regular suspend/resume there is no observable difference vs. sched
- * clock, but it might affect some of the nasty low level debug printks.
- *
- * OTOH, access to sched clock is not guaranteed across suspend/resume on
- * all systems either so it depends on the hardware in use.
- *
- * If that turns out to be a real problem then this could be mitigated by
- * using sched clock in a similar way as during early boot. But it's not as
- * trivial as on early boot because it needs some careful protection
- * against the clock monotonic timestamp jumping backwards on resume.
- */
-void ktime_get_fast_timestamps(struct ktime_timestamps *snapshot)
-{
-	struct timekeeper *tk = &tk_core.timekeeper;
-
-	snapshot->real = __ktime_get_real_fast(&tk_fast_mono, &snapshot->mono);
-	snapshot->boot = snapshot->mono + ktime_to_ns(data_race(tk->offs_boot));
-}
-
 /**
  * halt_fast_timekeeper - Prevent fast timekeeper from accessing clocksource.
  * @tk: Timekeeper to snapshot.
-- 
2.47.1