Re: [Bulk] Re: [Bulk] Re: [patch 2.6.18] genirq: remove oops with fasteoi irq_chip descriptors

From: David Brownell
Date: Wed Sep 27 2006 - 21:40:58 EST

Hi Thomas,

On Wednesday 27 September 2006 6:18 pm, Thomas Gleixner wrote:
> Dave,
> On Wed, 2006-09-27 at 17:39 -0700, David Brownell wrote:
> > - It wouldn't use chip->mask_ack() when that exists and those
> > other two routines don't, even though mask_ack_irq() is a
> > conveniently defined inline.
> So why not replace it by mask_ack_irq() ?

It'd still oops on chips with just enable(), disable(), and eoi().

Which, on my brief scan of the codebase, appears to be one of the
accepted ways to craft a fasteoi irq_chip.

> > - Umm, how could it ever be correct to leave the IRQ active
> > without a dispatcher? ISTR the rationale for that delayed
> > disable was not purely to be a PITA for all driver writers,
> > but was to address some issue with edge triggering. In that
> > path, triggering was no longer to be allowed ...
> Your patch would result in default_disable() when no shutdown function
> is provided. default_disable() does the delayed disable thing, while you
> remove the handler. The next event on that line will cause a spurious
> IRQ.

That may be an argument that the default shutdown() should not be the
same as the default disable(). Unless shutdown() is going away??

I still dislike that delayed disable() mechanism. Every time I've
seen ("tripped over") it in action it's been the cause of bugs.

> > - Plus ack()ing the IRQ there just seemed pretty dubious. It's
> > not like there would be anything preventing that signal line
> > from being lowered (or raised, etc) immediately after the ack(),
> > which in some hardware would latch the IRQ until later unmask().
> >
> > Leaving the question: what's the point of it?? The overall
> > system has to behave sanely with or without the ack(); just
> > clearing a latch doesn't mean it couldn't get set later.
> Fair enough.
> > > > So what's the correct fix then ... use enable() and disable()?
> > > > Oopsing isn't OK...
> > >
> > > True, but we can not unconditionally change the semantics.
> >
> > Some current semantics are "it oopses". That's a good definition
> > of semantics that _must_ be changed. We're not Microsoft. ;)
> Agreed, it just depends on how they get fixed.

I thought maybe submitting a reasonably sane patch would be the
best way to start that discussion. :)

The only issue appears to be how that rarely-used "get rid of
the handler" code path should work.

> > > Does it break existing or new code ?
> >
> > Could any code relying on those previous semantics have been
> > correct in the first place, though? Seemed to me it couldn't
> > have been.
> >
> > Plus, unregistering IRQ dispatchers is a strange notion. I've
> > never seen it done in practice ... normally, they get set up once
> > during chip/board setup then never changed. Bugs in code paths
> > like that have been known to last for decades unfixed.
> Agreed. Nothing is using this currently.

Aha! So if it's "nothing" then that rarely/not-used path can change
without negative impact...

> > > Sorry, I did not think about the defaults in the first place. The
> > > conditionals in manage,c are probably superflous leftovers from one of
> > > the evolvement.
> >
> > And that's how I was taking that particular mask() then ack() too,
> > especially given it never used mask_ack() when it should have, and
> > since that logic oopsed in various cases with fasteoi handlers.
> The remaining question is whether mask_ack_irq() or shutdown() is the
> correct approach. Your patch would make it mandatory to implement
> shutdown at least for such removable stuff.

Well, an implementation of shutdown() _is_ always provided. At least
now; I don't have time to track your MM patches.

> I'm not sure about that right now as I'm too tired.

I expect that after you sleep on this, something will come to mind. ;)

- Dave

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