[PATCH v7 0/1] pps: improve PREEMPT_RT performance

[Calvin Owens <calvin@wbinvd.org>]

Hello all,

I'm relaying v7 for Michael.

A quick note, this conflicts with the patch I have out to remove
capture_clear: it's trivial to resolve, but if it saves anybody time let
me know and I can respin one or the other.

Thanks,
Calvin

---
From: Michael Byczkowski <by@by-online.de>

Changes since v6: https://lore.kernel.org/lkml/cover.1779733602.git.calvin@wbinvd.org/
 - patches 2 and 3 are dropped since neither lock is ever taken in
   hardirq context.

Changes since v5: https://lore.kernel.org/lkml/719A31CE-CA58-45C3-A013-1BFE81F724C5@by-online.de/
 - Reordered: the pps_kc_hardpps_lock conversion now precedes the
   pps_device.lock conversion. The previous order would have briefly
   produced a raw_spinlock holding a sleeping spinlock on PREEMPT_RT
   (Sebastian).
 - Patch 1/3: commit message reworded to describe the handler split
   structurally first, then its PREEMPT_RT benefit (Sebastian).
 - Patch 2/3: refactored pps_kc_bind() and pps_kc_remove() to use
   guard(raw_spinlock_irq) for scope-based lock release. Eliminates
   four duplicated unlock call sites in pps_kc_bind() and the
   ambiguous bracket structure that resulted from them (Sebastian).
 - Rodolfo's Acked-by on patch 2/3 is preserved from v5; the guard()
   refactor is purely stylistic and was suggested by Sebastian, but
   please re-ack or NAK if disagreement.

Changes since v4: https://lore.kernel.org/lkml/B24484C5-3117-4C56-9522-1EE9876E64BA@by-online.de/
 - Patch 2/3: added Acked-by: Rodolfo Giometti <giometti@enneenne.com>

Changes since v3: https://lore.kernel.org/lkml/83318241-44C3-48BE-829D-5C5F82A78A74@by-online.de/
 - Patch 2/3: fixed lost indentation on pps_kc_event() call
   (reported by Rodolfo Giometti <giometti@enneenne.com>)

Changes since v2: https://lore.kernel.org/lkml/1BB87C0C-33C1-45C3-B50E-C5F349DA3FDC@by-online.de/
 - 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@buchwitz.com>)

This patchset addresses three sources of PPS 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).

Tested on a Raspberry Pi 5 running 7.0.1 and 7.1-rc PREEMPT_RT kernels.
On non-RT kernels there is zero behavioral change.

Michael Byczkowski (1):
  pps: pps-gpio: split IRQ handler into hardirq timestamper + threaded
    handler

 drivers/pps/clients/pps-gpio.c | 37 +++++++++++++++++++++++-----------
 1 file changed, 25 insertions(+), 12 deletions(-)

-- 
2.47.3