Re: [PATCH 2/6] brcmfmac: Handling the interrupt in ISRdirectly for non-OOB

[Franky Lin]

On 08/28/2012 04:13 AM, Wei Ni wrote:
> On Tue, 2012-08-28 at 04:06 +0800, Stephen Warren wrote:
> > On 08/27/2012 09:24 AM, Arend van Spriel wrote:
> > > On 08/27/2012 12:25 PM, Wei Ni wrote:
> > > > In case of inband interrupts, if we handle the interrupt in dpc thread,
> > > > two level of thread switching takes place to process wifi interrupts.
> > > > One in SDHCI driver and the other in Wifi driver. This may cause the
> > > > system
> > > > instability.
> > >
> > > Looking into the sdhci/mmc code indeed shows that the brcmfmac irq
> > > handler is not called in true IRQ context. So the dpc thread may add
> > > unnecessary complexity, but to me there is not indication that there is
> > > a stability issue.
>
> The brcmfmac irq handler is called in the thread sdio_irq_thread(), this
> thread indeed is driven by the sdhci irq, although it's not the true IRQ
> context. If the brcmfmac doesn't clear the IRQ condition ASAP, the
> sdio_irq_thread will be triggered again and again, and in this condition
> it's too difficult to run the brcmfmac dpc thread, more and more
> interrupt can't be handled.
>
> > > > Because the SDHCI calls sdio_irq_thread() to handle the irq, this
> > > > thread locks
> > > > mmc host and calls wifi handler. It expects WiFi handler to be quick and
> > > > enables sdio interrupt from card at end. If wifi handler defers this
> > > > work for
> > > > a different thread, sdio_irq_thread() will be stuck on next wifi
> > > > interrupt
> > > > since mmc lock is not freed.
> > >
> > > Not sure if I can follow this explanation. The isr is called with host
> > > claimed (by sdio_irq_thread) and all it does is at a linked list member
> > > and signal the dpc thread. After doing this the host is released.
> >
> > Is the issue something like the ISR handler or first level of threading
> > does:
> >
> > * Trigger DPC
> > * Re-enable interrupt
> >
> > So that the interrupt then fires again before the triggered DPC can run
> > to handle/clear it, thus causing an interrupt storm?
> >
> > Whereas handling the interrupt directly prevents this race condition?
>
> Above is my understanding.

Hi Wei,

I understand the issue here and totally agree that we should treat 
in-band and out-band interrupts differently. But my concern is that the 
behavior of releasing the host before calling brcmf_sdbrcm_isr and grab 
it after is likely error prone. Also we are restructuring the dpc 
routine internally and it's almost done. I will find a better solution 
for in-band interrupt and get it the queue as well. So I suggest 
dropping this patch.

Thanks,
Franky

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