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 11:54:11 +0530
From:	Sekhar Nori <nsekhar@...com>
To:	Joel Fernandes <agnel.joel@...il.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 Monday 12 August 2013 09:59 AM, Joel Fernandes wrote:
> 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.

Okay, I wont queue for -rc then. If Vinod wants to take this along with
rest of the series, you can add my:

Acked-by: Sekhar Nori <nsekhar@...com>

Thanks,
Sekhar
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