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Date: Thu, 20 May 2010 10:14:02 +0200 (CEST)
From: Thomas Gleixner <tglx@linutronix.de>
To: Jan-Bernd Themann <THEMANN@de.ibm.com>
cc: michael@ellerman.id.au, Brian King <brking@linux.vnet.ibm.com>,
       Doug Maxey <doug.maxey@us.ibm.com>, dvhltc@linux.vnet.ibm.com,
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       linuxppc-dev@lists.ozlabs.org, niv@linux.vnet.ibm.com,
       Will Schmidt <will_schmidt@vnet.ibm.com>
Subject: Re: [PATCH RT] ehea: make receive irq handler non-threaded
 (IRQF_NODELAY)
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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