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

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

[Calvin Owens]

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

[PATCH v7 1/1] pps: pps-gpio: split IRQ handler into hardirq timestamper + threaded handler

[Calvin Owens]

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

Split the pps-gpio interrupt handler into a primary (hardirq) handler
that captures the PPS timestamp at interrupt entry, and a threaded
handler that processes the event. This produces the same two-part
handler structure on both PREEMPT_RT and non-RT kernels.

On non-RT kernels the threaded portion runs immediately after the
primary, with no behavioral change compared to the previous
single-handler implementation.

On PREEMPT_RT, where interrupt handlers are force-threaded by default,
the previous single-handler implementation captured the timestamp
inside the threaded portion, after IRQ-thread scheduling delay. With
the split, the timestamp is captured in true hardirq context as it is
on non-RT kernels, eliminating a significant source of PPS jitter on
RT systems.

Tested-by: Michael Byczkowski <by@by-online.de>
Tested-by: Calvin Owens <calvin@wbinvd.org>
Acked-by: Rodolfo Giometti <giometti@enneenne.com>
Signed-off-by: Michael Byczkowski <by@by-online.de>
Signed-off-by: Calvin Owens <calvin@wbinvd.org>
---
 drivers/pps/clients/pps-gpio.c | 37 +++++++++++++++++++++++-----------
 1 file changed, 25 insertions(+), 12 deletions(-)

diff --git a/drivers/pps/clients/pps-gpio.c b/drivers/pps/clients/pps-gpio.c
index 935da68610c7..ed111621ee5f 100644
--- a/drivers/pps/clients/pps-gpio.c
+++ b/drivers/pps/clients/pps-gpio.c
@@ -35,33 +35,44 @@ struct pps_gpio_device_data {
 	bool capture_clear;
 	unsigned int echo_active_ms;	/* PPS echo active duration */
 	unsigned long echo_timeout;	/* timer timeout value in jiffies */
+	struct pps_event_time ts;	/* timestamp captured in hardirq */
 };
 
 /*
  * Report the PPS event
  */
 
-static irqreturn_t pps_gpio_irq_handler(int irq, void *data)
+/*
+ * Primary hardirq handler -- runs in hardirq context even on PREEMPT_RT.
+ * Only captures the timestamp; all other work is deferred to the thread.
+ */
+static irqreturn_t pps_gpio_irq_hardirq(int irq, void *data)
 {
-	const struct pps_gpio_device_data *info;
-	struct pps_event_time ts;
-	int rising_edge;
+	struct pps_gpio_device_data *info = data;
+
+	pps_get_ts(&info->ts);
 
-	/* Get the time stamp first */
-	pps_get_ts(&ts);
+	return IRQ_WAKE_THREAD;
+}
 
-	info = data;
+/*
+ * Threaded handler -- processes the PPS event using the timestamp
+ * captured in hardirq context above.
+ */
+static irqreturn_t pps_gpio_irq_thread(int irq, void *data)
+{
+	struct pps_gpio_device_data *info = data;
+	int rising_edge;
 
-	/* Small trick to bypass the check on edge's direction when capture_clear is unset */
 	rising_edge = info->capture_clear ?
 		      gpiod_get_value(info->gpio_pin) : !info->assert_falling_edge;
 	if ((rising_edge && !info->assert_falling_edge) ||
 			(!rising_edge && info->assert_falling_edge))
-		pps_event(info->pps, &ts, PPS_CAPTUREASSERT, data);
+		pps_event(info->pps, &info->ts, PPS_CAPTUREASSERT, data);
 	else if (info->capture_clear &&
 			((rising_edge && info->assert_falling_edge) ||
 			(!rising_edge && !info->assert_falling_edge)))
-		pps_event(info->pps, &ts, PPS_CAPTURECLEAR, data);
+		pps_event(info->pps, &info->ts, PPS_CAPTURECLEAR, data);
 	else
 		dev_warn_ratelimited(&info->pps->dev, "IRQ did not trigger any PPS event\n");
 
@@ -210,8 +221,10 @@ static int pps_gpio_probe(struct platform_device *pdev)
 	}
 
 	/* register IRQ interrupt handler */
-	ret = request_irq(data->irq, pps_gpio_irq_handler,
-			  get_irqf_trigger_flags(data), data->info.name, data);
+	ret = request_threaded_irq(data->irq,
+			  pps_gpio_irq_hardirq, pps_gpio_irq_thread,
+			  get_irqf_trigger_flags(data) | IRQF_ONESHOT,
+			  data->info.name, data);
 	if (ret) {
 		pps_unregister_source(data->pps);
 		dev_err(dev, "failed to acquire IRQ %d\n", data->irq);
-- 
2.47.3

Re: [PATCH v7 1/1] pps: pps-gpio: split IRQ handler into hardirq timestamper + threaded handler

[Sebastian Andrzej Siewior]

On 2026-06-01 17:44:09 [-0700], Calvin Owens wrote:
> From: Michael Byczkowski <by@by-online.de>
> 
> Split the pps-gpio interrupt handler into a primary (hardirq) handler
> that captures the PPS timestamp at interrupt entry, and a threaded
> handler that processes the event. This produces the same two-part
> handler structure on both PREEMPT_RT and non-RT kernels.
> 
> On non-RT kernels the threaded portion runs immediately after the
> primary, with no behavioral change compared to the previous
> single-handler implementation.
> 
> On PREEMPT_RT, where interrupt handlers are force-threaded by default,
> the previous single-handler implementation captured the timestamp
> inside the threaded portion, after IRQ-thread scheduling delay. With
> the split, the timestamp is captured in true hardirq context as it is
> on non-RT kernels, eliminating a significant source of PPS jitter on
> RT systems.
> 
> Tested-by: Michael Byczkowski <by@by-online.de>
> Tested-by: Calvin Owens <calvin@wbinvd.org>
> Acked-by: Rodolfo Giometti <giometti@enneenne.com>
> Signed-off-by: Michael Byczkowski <by@by-online.de>
> Signed-off-by: Calvin Owens <calvin@wbinvd.org>
> ---

Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>

Sebastian

Re: [PATCH v7 1/1] pps: pps-gpio: split IRQ handler into hardirq timestamper + threaded handler

[Michael Byczkowski]

Thank you Sebastian, and thank you Calvin for relaying. I appreciate the
thorough review that got this to the right shape.

Best regards,
	Michael


> On 2. Jun 2026, at 08:36, Sebastian Andrzej Siewior <bigeasy@linutronix.de> wrote:
> 
> On 2026-06-01 17:44:09 [-0700], Calvin Owens wrote:
>> From: Michael Byczkowski <by@by-online.de>
>> 
>> Split the pps-gpio interrupt handler into a primary (hardirq) handler
>> that captures the PPS timestamp at interrupt entry, and a threaded
>> handler that processes the event. This produces the same two-part
>> handler structure on both PREEMPT_RT and non-RT kernels.
>> 
>> On non-RT kernels the threaded portion runs immediately after the
>> primary, with no behavioral change compared to the previous
>> single-handler implementation.
>> 
>> On PREEMPT_RT, where interrupt handlers are force-threaded by default,
>> the previous single-handler implementation captured the timestamp
>> inside the threaded portion, after IRQ-thread scheduling delay. With
>> the split, the timestamp is captured in true hardirq context as it is
>> on non-RT kernels, eliminating a significant source of PPS jitter on
>> RT systems.
>> 
>> Tested-by: Michael Byczkowski <by@by-online.de>
>> Tested-by: Calvin Owens <calvin@wbinvd.org>
>> Acked-by: Rodolfo Giometti <giometti@enneenne.com>
>> Signed-off-by: Michael Byczkowski <by@by-online.de>
>> Signed-off-by: Calvin Owens <calvin@wbinvd.org>
>> ---
> 
> Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> 
> Sebastian