Re: [PATCH] pps: improve PREEMPT_RT performance

[Michael Byczkowski]

Dear Calvin,

Thank you very much for your kind words, will do.

Best regards,
	Michael


> On 6. Apr 2026, at 19:24, Calvin Owens <calvin@xxxxxxxxxx> wrote:
> 
> On Monday 04/06 at 14:11 +0200, Michael Byczkowski wrote:
>> 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.
> 
> Hi Michael,
> 
> Thanks for working on this, and for the nice clean patches :)
> 
> Your series is happily running on my pps-gpio setup. When you resend
> the patches, feel free to add:
> 
> Tested-by: Calvin Owens <calvin@xxxxxxxxxx>
> 
> One quick thought below:
> 
>> 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
> 
> These should be combined into one commit to avoid bisection problems,
> the intermediate states don't compile. But maybe you already intend to
> do that, I see you've grouped them together here.
> 
> Cheers,
> Calvin