Re: [PATCH] PCI: let pci_request_irq properly deal with threaded interrupts

[Marc Zyngier <marc.zyngier@arm.com>]

On Mon, 30 Jul 2018 23:36:57 +0100,
Thomas Gleixner <tglx@linutronix.de> wrote:
> 
> On Mon, 30 Jul 2018, Bjorn Helgaas wrote:
> 
> > [+cc Thomas, Christoph, LKML]
> 
> + Marc
> 
> > On Mon, Jul 30, 2018 at 12:03:42AM +0200, Heiner Kallweit wrote:
> > > If we have a threaded interrupt with the handler being NULL, then
> > > request_threaded_irq() -> __setup_irq() will complain and bail out
> > > if the IRQF_ONESHOT flag isn't set. Therefore check for the handler
> > > being NULL and set IRQF_ONESHOT in this case.
> > > 
> > > This change is needed to migrate the mei_me driver to
> > > pci_alloc_irq_vectors() and pci_request_irq().
> > > 
> > > Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
> > 
> > I'd like an ack from Thomas because this requirement about IRQF_ONESHOT
> > usage isn't mentioned in the request_threaded_irq() function doc or
> > Documentation/
> 
> Right. The documentation really needs some love and care. :(
> 
> Yes, request for pure threaded interrupts are rejected if the oneshot flag
> is not set. The reason is that this would be deadly especially with level
> triggered interrupts because the primary default handler just wakes the
> thread and then reenables interrupts, which will make the interrupt come
> back immediately and the thread won't have a chance to actually shut it up
> in the device.
> 
> That made me look into that code again and I found that we added a flag for
> irq chips to tell the core that the interrupt is one shot safe, i.e. that
> it can be requested w/o IRQF_ONESHOT. That was initially added to optimize
> MSI based interrupts which are oneshot safe by implementation.
> 
>   dc9b229a58dc ("genirq: Allow irq chips to mark themself oneshot safe")
> 
> The original patch added that flag to the x86 MSI irqchip code, but that
> part was not applied for reasons which slipped from memory. It might be
> worthwhile to revisit that in order to avoid the mask/unmask overhead for
> such cases.

Yup, that would actually be beneficial to a range of interrupt
controllers (only an obscure GPIO driver makes use of this flag).

We could also consider extending this to support interrupt
hierarchies, as __setup_irq() seems only concerned with the top of the
stack (an IRQ provided by a generic MSI stack and backed by an irqchip
providing IRQCHIP_ONESHOT_SAFE would go unnoticed).

	M.

-- 
Jazz is not dead, it just smell funny.