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

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Message-ID: <CAD=GYpa75vqz120O2McxTXsPCWFb+RsO+7dZ4WRNQJgb3W=E=A@mail.gmail.com>
Date:	Sun, 11 Aug 2013 23:29:33 -0500
From:	Joel Fernandes <agnel.joel@...il.com>
To:	Sekhar Nori <nsekhar@...com>
Cc:	Joel Fernandes <joelf@...com>, 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 Sun, Aug 11, 2013 at 11:25 PM, Sekhar Nori <nsekhar@...com> wrote:
> 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.

The race was uncovered by recent EDMA patch series, So this patch can
go in for next kernel release as such, I am not aware of any other DMA
user that maybe uncovering the race condition.

Thanks,

-Joel
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