[PATCH v1] i3c: master: allocate IBI workqueue with WQ_HIGHPRI

[PATCH v1] i3c: master: allocate IBI workqueue with WQ_HIGHPRI

[Stanley Chu]

From: Stanley Chu <yschu@nuvoton.com>

The IBI (In-Band Interrupt) workqueue is allocated with only
WQ_MEM_RECLAIM, which places IBI payload processing at normal
worker priority. This is inadequate given the time-sensitive
nature of IBI handling.

In the I3C protocol, when a target asserts an IBI, the SDA line
is held low until the master acknowledges and completes the
exchange. The IRQ handler (top half) ACKs the IBI, reads the
payload, emits a STOP, and immediately queues the payload
processing to the per-device ordered workqueue via
i3c_master_queue_ibi() — effectively the bottom half of the
IBI interrupt path.

If this workqueue worker is delayed by competing normal-priority
tasks, the IBI notification reaches the client driver late. For
latency-sensitive clients (e.g. sensors reporting alerts,
hotplug events), this defeats the purpose of using IBI over
polling. Furthermore, because the ordered workqueue serialises
slots, a backlog of delayed slots can exhaust the pre-allocated
IBI slot pool, causing subsequent IBIs to be dropped at the
hardware level.

Add WQ_HIGHPRI to ensure IBI bottom-half work is scheduled
promptly after the top-half IRQ handler enqueues it, keeping
the IBI processing pipeline consistent with the interrupt-like
semantics the protocol demands.

Signed-off-by: Stanley Chu <yschu@nuvoton.com>
---
 drivers/i3c/master.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/i3c/master.c b/drivers/i3c/master.c
index f1be38a640ca..8fdd67a031ff 100644
--- a/drivers/i3c/master.c
+++ b/drivers/i3c/master.c
@@ -3505,7 +3505,8 @@ int i3c_dev_request_ibi_locked(struct i3c_dev_desc *dev,
 	if (!ibi)
 		return -ENOMEM;
 
-	ibi->wq = alloc_ordered_workqueue(dev_name(i3cdev_to_dev(dev->dev)), WQ_MEM_RECLAIM);
+	ibi->wq = alloc_ordered_workqueue(dev_name(i3cdev_to_dev(dev->dev)),
+					  WQ_MEM_RECLAIM | WQ_HIGHPRI);
 	if (!ibi->wq) {
 		kfree(ibi);
 		return -ENOMEM;
-- 
2.34.1

Re: [PATCH v1] i3c: master: allocate IBI workqueue with WQ_HIGHPRI

[Mukesh Savaliya]

On 8/10/2026 7:00 AM, Stanley Chu wrote:
> From: Stanley Chu <yschu@nuvoton.com>
> 
> The IBI (In-Band Interrupt) workqueue is allocated with only
> WQ_MEM_RECLAIM, which places IBI payload processing at normal
> worker priority. This is inadequate given the time-sensitive
> nature of IBI handling.
> 
> In the I3C protocol, when a target asserts an IBI, the SDA line
> is held low until the master acknowledges and completes the
> exchange. The IRQ handler (top half) ACKs the IBI, reads the
> payload, emits a STOP, and immediately queues the payload
> processing to the per-device ordered workqueue via
> i3c_master_queue_ibi() — effectively the bottom half of the
> IBI interrupt path.
> 
> If this workqueue worker is delayed by competing normal-priority
> tasks, the IBI notification reaches the client driver late. For
> latency-sensitive clients (e.g. sensors reporting alerts,
> hotplug events), this defeats the purpose of using IBI over
> polling. Furthermore, because the ordered workqueue serialises
> slots, a backlog of delayed slots can exhaust the pre-allocated
> IBI slot pool, causing subsequent IBIs to be dropped at the
> hardware level.
> 
> Add WQ_HIGHPRI to ensure IBI bottom-half work is scheduled
> promptly after the top-half IRQ handler enqueues it, keeping
> the IBI processing pipeline consistent with the interrupt-like
> semantics the protocol demands.
> 
> Signed-off-by: Stanley Chu <yschu@nuvoton.com>
> ---

Acked-by: Mukesh Kumar Savaliya <mukesh.savaliya@oss.qualcomm.com>

Re: [PATCH v1] i3c: master: allocate IBI workqueue with WQ_HIGHPRI

[Alexandre Belloni]

On Mon, 10 Aug 2026 09:30:59 +0800, Stanley Chu wrote:
> The IBI (In-Band Interrupt) workqueue is allocated with only
> WQ_MEM_RECLAIM, which places IBI payload processing at normal
> worker priority. This is inadequate given the time-sensitive
> nature of IBI handling.
> 
> In the I3C protocol, when a target asserts an IBI, the SDA line
> is held low until the master acknowledges and completes the
> exchange. The IRQ handler (top half) ACKs the IBI, reads the
> payload, emits a STOP, and immediately queues the payload
> processing to the per-device ordered workqueue via
> i3c_master_queue_ibi() — effectively the bottom half of the
> IBI interrupt path.
> 
> [...]

Applied, thanks!

[1/1] i3c: master: allocate IBI workqueue with WQ_HIGHPRI
      https://git.kernel.org/i3c/c/591980aaaa46

Best regards,

-- 
Alexandre Belloni, co-owner and COO, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com