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

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Date:	Thu, 20 May 2010 10:14:02 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Jan-Bernd Themann <THEMANN@...ibm.com>
cc:	michael@...erman.id.au, 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,
	linuxppc-dev@...ts.ozlabs.org, niv@...ux.vnet.ibm.com,
	Will Schmidt <will_schmidt@...t.ibm.com>
Subject: Re: [PATCH RT] ehea: make receive irq handler non-threaded
 (IRQF_NODELAY)

On Thu, 20 May 2010, Jan-Bernd Themann wrote:
> > > Thought more about that. The case at hand (ehea) is nasty:
> > >
> > > The driver does _NOT_ disable the rx interrupt in the card in the rx
> > > interrupt handler - for whatever reason.
> >
> > Yeah I saw that, but I don't know why it's written that way. Perhaps
> > Jan-Bernd or Doug will chime in and enlighten us? :)
> 
> From our perspective there is no need to disable interrupts for the
> RX side as the chip does not fire further interrupts until we tell
> the chip to do so for a particular queue. We have multiple receive

The traces tell a different story though:

    ehea_recv_irq_handler()
      napi_reschedule()
    eoi()
    ehea_poll()
      ...
      ehea_recv_irq_handler()    <---------------- ???
        napi_reschedule()
      ...
      napi_complete()

Can't tell whether you can see the same behaviour in mainline, but I
don't see a reason why not.

> queues with an own interrupt each so that the interrupts can arrive
> on multiple CPUs in parallel.  Interrupts are enabled again when we
> leave the NAPI Poll function for the corresponding receive queue.

I can't see a piece of code which does that, but that's probably just
lack of detailed hardware knowledge on my side.

Thanks,

	tglx
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