linux-kernel - Re: [PATCH] mailbox: qcom-ipcc: flag IRQ NO

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Message-ID: <Yzv/3vsIMtgu7noY@linutronix.de>
Date:   Tue, 4 Oct 2022 11:41:50 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Eric Chanudet <echanude@...hat.com>
Cc:     Andy Gross <agross@...nel.org>,
        Bjorn Andersson <andersson@...nel.org>,
        Manivannan Sadhasivam <mani@...nel.org>,
        Jassi Brar <jassisinghbrar@...il.com>,
        linux-arm-msm@...r.kernel.org, linux-rt-users@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mailbox: qcom-ipcc: flag IRQ NO_THREAD

On 2022-10-03 13:08:49 [-0400], Eric Chanudet wrote:
> PREEMPT_RT forces qcom-ipcc's handler to be threaded with interrupts
> enabled, which triggers a warning in __handle_irq_event_percpu():
>     irq 173 handler irq_default_primary_handler+0x0/0x10 enabled interrupts
>     WARNING: CPU: 0 PID: 77 at kernel/irq/handle.c:161 __handle_irq_event_percpu+0x4c4/0x4d0
> 
> Mark it IRQF_NO_THREAD to avoid running the handler in a threaded
> context with threadirqs or PREEMPT_RT enabled.

The important bit of information is that this is a IRQ-multiplexer and
such it must not be threaded. Otherwise its child-interrupts would be
invoked from the thread handler which is not desired.
This is noticed by PREEMPT_RT but also on a non-PREEMPT_RT kernel where
`threadirqs' has been used.

Side note: Using request_irq() has the side effect that this interrupt
pops-up in /proc/interrupts and the "child-interrupt", too. So you
account two interrupts while "one" on the HW side occurred. 
Maybe irq_set_chained_handler_and_data() would be better use.
Either way, this addresses the problem,

Acked-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>

> Signed-off-by: Eric Chanudet <echanude@...hat.com>

Sebastian