linux-kernel - Re: [PATCH 3/4] media: dw100: Fix kernel oops with PREEMPT

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Message-ID: <20260123082405.yx7thGtq@linutronix.de>
Date: Fri, 23 Jan 2026 09:24:05 +0100
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: Stefan Klug <stefan.klug@...asonboard.com>
Cc: Laurent Pinchart <laurent.pinchart@...asonboard.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Nicolas Dufresne <nicolas@...fresne.ca>,
	Xavier Roumegue <xavier.roumegue@....nxp.com>,
	Mauro Carvalho Chehab <mchehab@...nel.org>,
	Clark Williams <clrkwllms@...nel.org>, linux-media@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-rt-devel@...ts.linux.dev
Subject: Re: [PATCH 3/4] media: dw100: Fix kernel oops with PREEMPT_RT enabled

On 2026-01-14 18:22:34 [+0100], Stefan Klug wrote:
> Hi Sebastian,
Hi,

sorry for being late…

> I did a bit more testing and got results that I fail to completely
> understand.
> 
> If I enable IRQF_ONESHOT and use the threaded_fn, on a non PREEMPT_RT
> system I regularly observe the timeout message.
> 
> If I pass irqflags=0 and use the hard handler on a PREEMPT_RT system I
> expected the same behavior (as the hard handler gets changed to be
> threaded and implicitely ONESHOT is set). But I don't see the timeout
> messages.

If you have the driver with the removed ONESHOT and boot !RT with
threadirqs then you should have the same behaviour. RT threads all
handler not just one. This might change the behaviour if all interrupts
are served by one CPU and some interrupts are handled prior and have an
effect on this (threaded) one.

> Is there anything else that I need to do on PREEMPT_RT to force the
> threaded behavior besides enabling the config? Or is the irq thread
> running with higher priority and therefore possibly faster?

In both configurations (RT and !RT with threaded interrupts) the
interrupt handed is running at SCHED_FIFO prio 50. This has a higher
priority than the "normal" userland which runs at SCHED_OTHER but the
same priority as the remaining threaded interrupts.

> Running irqflags=0 and the hard handler on a non PREEMPT_RT system
> didn't have any negative side effects. So maybe that is really the
> solution...
> 
> I'll ping Xavier if he has more details on the hardware.
> 
> Best regards,
> Stefan

Sebastian