[PATCH] pps: improve PREEMPT_RT performance

[Michael Byczkowski]

Dear Rodolfo,

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 on a Rapsberry Pi HAT providing PPS via GPIO.

I found three areas 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).
  On non-RT kernels, request_threaded_irq with an explicit primary
  handler behaves identically to the current code.

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, which compiles to identical code on non-RT.

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 with a PPS-disciplined NTP server. They apply cleanly
against mainline. On non-RT kernels, raw_spinlock_t compiles identically
to spinlock_t, and request_threaded_irq with a primary handler works
the same as request_irq — so there is zero behavioral change for
non-RT users.

The patches are available as individual commits at:
 https://github.com/by/linux-PPS

[PATCH 1/3] pps: pps-gpio: split handler into hardirq timestamp and threaded processing
https://github.com/by/linux-PPS/commit/e811a4e6f63f39a782db2a2fe1588419de96275b

[PATCH 2/3] pps: convert pps_device lock to raw_spinlock for PREEMPT_RT
(covers include/linux/pps_kernel.h, drivers/pps/kapi.c, drivers/pps/pps.c)
https://github.com/by/linux-PPS/commit/918e6f9c87adc552bda3b9b5847eb535f943c68e
https://github.com/by/linux-PPS/commit/d1eb9d80768f6648ff4638b5f83028344f0860e9
https://github.com/by/linux-PPS/commit/1f149bf2c449a36730adb341fdf626ed07953f5f

[PATCH 3/3] pps: convert pps_kc_hardpps_lock to raw_spinlock for PREEMPT_RT
https://github.com/by/linux-PPS/commit/a37a12ed96c8d5d0d73ac9e8f43cf0118cc72c1c

Files changed:
 drivers/pps/clients/pps-gpio.c   (patch 1)
 include/linux/pps_kernel.h       (patch 2)
 drivers/pps/kapi.c               (patch 2)
 drivers/pps/pps.c                (patch 2)
 drivers/pps/kc.c                 (patch 3)

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

And please forgive my complete lack of erperience in the kernel patch process, thanks.

Best regards,
	Michael