linux-kernel - Re: [PATCH RT] ehea: make receive irq handler non-threaded (IRQF

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Date:	Mon, 20 Sep 2010 16:26:22 +0200
From:	Jan-Bernd Themann <THEMANN@...ibm.com>
To:	Milton Miller <miltonm@....com>
Cc:	Brian King <brking@...ux.vnet.ibm.com>,
	Doug Maxey <doug.maxey@...ibm.com>, dvhltc@...ux.vnet.ibm.com,
	Darren Hart <dvhltc@...ibm.com>, linux-kernel@...r.kernel.org,
	linux-kernel-owner@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
	Michael Ellerman <michael@...erman.id.au>,
	Milton Miller <miltonm@....com>, niv@...ux.vnet.ibm.com,
	Thomas Gleixner <tglx@...utronix.de>,
	Will Schmidt <will_schmidt@...t.ibm.com>
Subject: Re: [PATCH RT] ehea: make receive irq handler non-threaded (IRQF_NODELAY)

Hi Milton,

sorry for the delayed answer, was on vacation.

linux-kernel-owner@...r.kernel.org wrote on 21.05.2010 11:18:01:
> linux-kernel-owner@...r.kernel.org
>
> On Thu, 20 May 2010 at 10:21:36 +0200 (CEST) Thomas Gleixner  wrote:
> > On Thu, 20 May 2010, Michael Ellerman wrote:
> > > On Wed, 2010-05-19 at 16:38 +0200, Thomas Gleixner wrote:
> > > > On Wed, 19 May 2010, Darren Hart wrote:
> > > >
> > > > > On 05/18/2010 06:25 PM, Michael Ellerman wrote:
> > > > > > On Tue, 2010-05-18 at 15:22 -0700, Darren Hart wrote:
> > >
> > > > > > The result of the discussion about two years ago on this was
that we
> > > > > > needed a custom flow handler for XICS on RT.
> > > > >
> > > > > I'm still not clear on why the ultimate solution wasn't to
> have XICS report
> > > > > edge triggered as edge triggered. Probably some complexity
> of the entire power
> > > > > stack that I am ignorant of.
> > > > >
> > > > > > Apart from the issue of loosing interrupts there is also
> the fact that
> > > > > > masking on the XICS requires an RTAS call which takes a global
lock.
> > > >
> > > > Right, I'd love to avoid that but with real level interrupts we'd
run
> > > > into an interrupt storm. Though another solution would be to issue
the
> > > > EOI after the threaded handler finished, that'd work as well, but
> > > > needs testing.
> > >
> > > Yeah I think that was the idea for the custom flow handler. We'd
reset
> > > the processor priority so we can take other interrupts (which the EOI
> > > usually does for you), then do the actual EOI after the handler
> > > finished.
> >
> > That only works when the card does not issue new interrupts until the
> > EOI happens. If the EOI is only relevant for the interrupt controller,
> > then you are going to lose any edge which comes in before the EOI as
> > well.
>
> Well, the real MSIs have an extra bit to allow the eoi to dally behind
> the mmio on another path and that should cover this race when the irq
> is left enabled.
>
> Jan-Bernd HEA has that change, right?

I don't now. We never hit problems so we did not look very deep into this
area.
We probably have to talk to the HEA HW developers to be sure.

Regards,
Jan-Bernd

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