Re: [PATCH 3/4] media: dw100: Fix kernel oops with PREEMPT_RT enabled

[Laurent Pinchart <laurent.pinchart@ideasonboard.com>]

On Mon, Jan 05, 2026 at 07:39:33PM -0500, Steven Rostedt wrote:
> On Tue, 6 Jan 2026 01:59:21 +0200 Laurent Pinchart wrote:
> 
> > > That's interesting, do you plan to update more drivers ? There is a lot of m2m
> > > using hard IRQ to minimize the idle time (save a context switch), but RT support
> > > might be more worth then that.  
> > 
> > This is a part of PREEMPT_RT that puzzles me. By turning regular
> > spinlocks into mutexes, RT seems to break drivers that use those
> > spinlocks in hard IRQ handlers. That's a very large number of drivers
> > given how widespread regular spinlock usage is. Do drivers need to be
> > manually converted to either raw spinlocks or threaded IRQ handlers ?
> 
> No. Pretty much all interrupts are converted into threaded interrupt
> handlers unless IRQF_NO_THREAD, IRQF_PERCPU, or IRQF_ONESHOT are specified.
> 
> The interrupt line is disabled until the thread handler is called.
> 
> > What about non-RT kernels, how can a driver avoid the thread scheduling
> > penalty in those cases, do they need to manually select between
> > request_irq() and request_threaded_irq() based on if RT is enabled ?
> > This puzzles me, it feels like I must be missing something.
> 
> The issue here is that the interrupt handler specifies ONESHOT which causes
> the handler to be executed in hard interrupt context.

Gotcha.

Stefan, please explain in the commit message why the ONESHOT flag is
set by the driver.

-- 
Regards,

Laurent Pinchart