[PATCH 0/5] mailbox: imx: Use threaded handler to avoid kworker in imx's remoteproc

[Sebastian Andrzej Siewior]

The imx's remoteproc driver uses a kworker from its mailbox callback to
complete the request. The reason is that the imx mailbox driver invokes
the callback from its interrupt handler and the remoteproc callback (at
least the rpmsg-tty) requires a preemptible context.

This works but is problematic in a PREEMPT_RT environment where the
latency of the invocation is important. By scheduling a kworker the
high task priority from the threaded handler is lost and the kworker
competes for CPU ressources with every SCHED_OTHER task in the system.
This can lead to long delays on a busy system with other RT threads
which are less important than the completion of this request.

Looking over other mailbox driver, like the arm_mhu for instance, they
use a threaded interrupt handler to invoke the callback. This avoids the
kworker detour.

The here suggested change utilises a threaded interrupt to invoke the
callback. The primary handler mask the interrupt source so that the
handler can run without getting interrupted by the interrupt again.
Doing so avoids marking the interrupt IRQF_ONESHOT so that that in a
shared-interrupt environment the other interrupt can still fire while
the first one is masked.

This change was tested on a im93 board with rpmsg-tty driver.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy at linutronix.de>
---
Sebastian Andrzej Siewior (5):
      mailbox: imx: Start splitting the IRQ handler in primary and threaded handler
      mailbox: imx: Move the RX part of the mailbox into the threaded handler
      mailbox: imx: Move the RXDB part of the mailbox into the threaded handler
      mailbox: imx: Don't force-thread the primary handler
      remoteproc: imx_rproc: Invoke the callback directly

 drivers/mailbox/imx-mailbox.c  | 65 +++++++++++++++++++++++++++++++++---------
 drivers/remoteproc/imx_rproc.c | 33 +--------------------
 2 files changed, 53 insertions(+), 45 deletions(-)
---
base-commit: 9e171fc1d7d7ab847a750c03571c87ac3c17bd84
change-id: 20260529-imx_mbox_rproc-7d512f5a6f78

Best regards,
-- 
Sebastian Andrzej Siewior <bigeasy at linutronix.de>