linux-kernel - Re: [PATCH 2/6] brcmfmac: Handling the interrupt in ISR directly for non-OOB

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Date:	Tue, 28 Aug 2012 19:13:33 +0800
From:	Wei Ni <wni@...dia.com>
To:	Stephen Warren <swarren@...dotorg.org>
CC:	Arend van Spriel <arend@...adcom.com>,
	"Franky (Zhenhui) Lin" <frankyl@...adcom.com>,
	"rvossen@...adcom.com" <rvossen@...adcom.com>,
	Rakesh Kumar <krakesh@...dia.com>,
	Laxman Dewangan <ldewangan@...dia.com>,
	"linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-wireless@...r.kernel.org" <linux-wireless@...r.kernel.org>,
	"brcm80211-dev-list@...adcom.com" <brcm80211-dev-list@...adcom.com>
Subject: Re: [PATCH 2/6] brcmfmac: Handling the interrupt in ISR directly
 for non-OOB

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.


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