linux-kernel - Re: [PATCH v3 02/12] ARM: edma: Don't clear EMR of channel in edma

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Date:	Mon, 12 Aug 2013 09:55:03 +0530
From:	Sekhar Nori <nsekhar@...com>
To:	Joel Fernandes <joelf@...com>
CC:	Mark Brown <broonie@...aro.org>, Tony Lindgren <tony@...mide.com>,
	Grant Likely <grant.likely@...retlab.ca>,
	Sricharan R <r.sricharan@...com>,
	Russell King <linux@....linux.org.uk>,
	Vinod Koul <vinod.koul@...el.com>,
	Lokesh Vutla <lokeshvutla@...com>, Chris Ball <cjb@...top.org>,
	Arnd Bergmann <arnd@...db.de>, Rajendra Nayak <rnayak@...com>,
	Rob Herring <rob.herring@...xeda.com>,
	Jason Kridner <jkridner@...gleboard.org>,
	Linux OMAP List <linux-omap@...r.kernel.org>,
	Linux ARM Kernel List <linux-arm-kernel@...ts.infradead.org>,
	Linux DaVinci Kernel List 
	<davinci-linux-open-source@...ux.davincidsp.com>,
	Balaji TK <balajitk@...com>,
	Linux MMC List <linux-mmc@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Santosh Shilimkar <santosh.shilimkar@...com>,
	Dan Williams <djbw@...com>, Olof Johansson <olof@...om.net>,
	Benoit Cousson <benoit.cousson@...aro.org>
Subject: Re: [PATCH v3 02/12] ARM: edma: Don't clear EMR of channel in edma_stop

On 8/8/2013 5:19 PM, Sekhar Nori wrote:
> On Monday 05 August 2013 09:44 PM, Joel Fernandes wrote:
>> We certainly don't want error conditions to be cleared any other
>> place but the EDMA error handler, as this will make us 'forget'
>> about missed events we might need to know errors have occurred.
>>
>> This fixes a race condition where the EMR was being cleared
>> by the transfer completion interrupt handler.
>>
>> Basically, what was happening was:
>>
>>             Missed event
>>              |
>>              |
>>              V
>> SG1-SG2-SG3-Null
>>          \
>>           \__TC Interrupt (Almost same time as ARM is executing
>> TC interrupt handler, an event got missed and also forgotten
>> by clearing the EMR).
>>
>> This causes the following  problems:
>>
>> 1.
>> If error interrupt is also pending and TC interrupt clears the EMR
>> by calling edma_stop as has been observed in the edma_callback function,
>> the ARM will execute the error interrupt even though the EMR is clear.
>> As a result, the  dma_ccerr_handler returns IRQ_NONE. If this happens
>> enough number of times, IRQ subsystem disables the interrupt thinking
>> its spurious which makes error handler never execute again.
>>
>> 2.
>> Also even if error handler doesn't return IRQ_NONE, the removing of EMR
>> removes the knowledge about which channel had a missed event, and thus
>> a manual trigger on such channels cannot be performed.
>>
>> The EMR is ultimately being cleared by the Error interrupt handler
>> once it is handled so we remove code that does it in edma_stop and
>> allow it to happen there.
>>
>> Signed-off-by: Joel Fernandes <joelf@...com>
> 
> Queuing this for v3.11 fixes. While committing, I changed the headline
> to remove capitalization and made it more readable by removing register
> level details. The new headline is:
> 
> ARM: edma: don't clear missed events in edma_stop()

Forgot to ask, should this be tagged for stable? IOW, how serious is
this race in current kernel (without the entire series applied)? I have
never observed it myself - so please provide details how easy/difficult
it is to hit this condition.

Thanks,
Sekhar
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