[PATCH 0/3] arm64: Allow early timestamping of kernel log

[Marc Zyngier]

So far, we've let the arm64 kernel start its meaningful time stamping
of the kernel log pretty late, which is caused by sched_clock() being
initialised rather late compared to other architectures.

Pavel Tatashin proposed[1] to move the initialisation of sched_clock
much earlier, which I had objections to. The reason for initialising
sched_clock late is that a number of systems have broken counters, and
we need to apply all kind of terrifying workarounds to avoid time
going backward on the affected platforms. Being able to identify the
right workaround comes pretty late in the kernel boot, and providing
an unreliable sched_clock, even for a short period of time, isn't an
appealing prospect.

To address this, I'm proposing that we allow an architecture to chose
to (1) divorce time stamping and sched_clock during the early phase of
booting, and (2) inherit the time stamping clock as the new epoch the
first time a sched_sched clock gets registered.

(1) would allow arm64 to provide a time stamping clock, however
unreliable it might be, while (2) would allow sched_clock to provide
time stamps that are continuous with the time-stamping clock.

The last patch in the series adds the necessary logic to arm64,
allowing the (potentially unreliable) time stamping of early kernel
messages.

Tested on a bunch of arm64 systems, both bare-metal and in VMs. Boot
tested on a x86 guest.

[1] https://lore.kernel.org/patchwork/patch/1015110/

Marc Zyngier (3):
  printk: Allow architecture-specific timestamping function
  sched/clock: Allow sched_clock to inherit timestamp_clock epoch
  arm64: Allow early time stamping

 arch/arm64/Kconfig          |  3 +++
 arch/arm64/kernel/setup.c   | 44 +++++++++++++++++++++++++++++++++++++
 include/linux/sched/clock.h | 13 +++++++++++
 kernel/printk/printk.c      |  4 ++--
 kernel/time/sched_clock.c   | 10 +++++++++
 5 files changed, 72 insertions(+), 2 deletions(-)

-- 
2.20.1