[PATCH v4 0/3] pps: improve PREEMPT_RT performance

[Michael Byczkowski]

Dear Rodolfo,

This is v4 of the PPS PREEMPT_RT patchset, with the fixe for the
sleeping-in-atomic issue in patch 2/3 as well as the lost indentation
now squashed in.

Changes since v3: - Patch 2/3: fixed lost indentation on pps_kc_event()
call (reported by Rodolfo Giometti <giometti@xxxxxxxxxxxx>)

Changes since v2: - Patch 2/3: moved wake_up_interruptible_all() and 
kill_fasync() out of raw_spinlock section to avoid sleeping-in-atomic on
PREEMPT_RT (reported by Nikolaus Buchwitz)

Changes since v2:
- Patch 2/3: moved wake_up_interruptible_all() and kill_fasync() out
 of raw_spinlock section to avoid sleeping-in-atomic on PREEMPT_RT
 (reported by Nikolaus Buchwitz <nb@xxxxxxxxxxxx>)

Andrew Morton pointed me your way as PPS maintainer. I'm running a
precision NTP time server on a Raspberry Pi 5 with a PREEMPT_RT kernel
and a u-blox ZED-F9P GPS receiver providing PPS via GPIO.

I found three issues in the PPS subsystem that cause unnecessary jitter
under PREEMPT_RT, while being fully backward-compatible with non-RT
kernels:

1. pps-gpio: The IRQ handler is force-threaded on PREEMPT_RT, so the
  PPS timestamp is captured after scheduling delay rather than at
  interrupt entry. Fix: split into a hardirq primary handler (captures
  timestamp only) and a threaded handler (processes the event).

2. pps_device.lock: spinlock_t becomes a sleeping mutex on PREEMPT_RT,
  allowing pps_event() to be preempted mid-update. Fix: convert to
  raw_spinlock_t and move sleeping calls out of the critical section.

3. pps_kc_hardpps_lock: Same issue as (2), in the kernel consumer path
  that calls hardpps(). Fix: convert to DEFINE_RAW_SPINLOCK.

All three patches are tested on a Raspberry Pi 5 running a 7.0.0-rc6
PREEMPT_RT kernel. On non-RT kernels there is zero behavioral change.

Signed-off-by: Michael Byczkowski <by@xxxxxxxxxxxx>
Tested-by: Michael Byczkowski <by@xxxxxxxxxxxx>

by (3):
  pps: pps-gpio: split IRQ handler into hardirq and threaded parts
  pps: convert pps_device lock to raw_spinlock for PREEMPT_RT
  pps: convert pps_kc_hardpps_lock to raw_spinlock for PREEMPT_RT

 drivers/pps/clients/pps-gpio.c | 37 +++++++++++++++++++++++-----------
 drivers/pps/kapi.c             | 18 ++++++++++-------
 drivers/pps/kc.c               | 22 ++++++++++----------
 drivers/pps/pps.c              | 16 +++++++--------
 include/linux/pps_kernel.h     |  2 +-
 5 files changed, 56 insertions(+), 39 deletions(-)

-- 
2.47.3