linux-kernel - Re: [PATCH 7/9] ARM: edma: Don't clear EMR of channel in edma

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Date:	Wed, 31 Jul 2013 20:59:27 -0500
From:	Joel Fernandes <joelf@...com>
To:	Sekhar Nori <nsekhar@...com>
CC:	Tony Lindgren <tony@...mide.com>,
	Santosh Shilimkar <santosh.shilimkar@...com>,
	Sricharan R <r.sricharan@...com>,
	Rajendra Nayak <rnayak@...com>,
	Lokesh Vutla <lokeshvutla@...com>,
	Matt Porter <matt@...orter.com>,
	Grant Likely <grant.likely@...retlab.ca>,
	Rob Herring <rob.herring@...xeda.com>,
	Vinod Koul <vinod.koul@...el.com>, Dan Williams <djbw@...com>,
	Mark Brown <broonie@...aro.org>,
	Benoit Cousson <benoit.cousson@...aro.org>,
	Russell King <linux@....linux.org.uk>,
	Arnd Bergmann <arnd@...db.de>, Olof Johansson <olof@...om.net>,
	Balaji TK <balajitk@...com>,
	Gururaja Hebbar <gururaja.hebbar@...com>,
	Chris Ball <cjb@...top.org>,
	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>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linux MMC List <linux-mmc@...r.kernel.org>
Subject: Re: [PATCH 7/9] ARM: edma: Don't clear EMR of channel in edma_stop

On 07/31/2013 04:35 AM, Sekhar Nori wrote:
> On Wednesday 31 July 2013 10:35 AM, Joel Fernandes wrote:
>> On 07/30/2013 03:29 AM, Sekhar Nori wrote:
>>> On Monday 29 July 2013 06:59 PM, Joel Fernandes wrote:
>>>> We certainly don't want error conditions to be cleared anywhere
>>>
>>> 'anywhere' is a really loaded term.
>>>
>>>> as this will make us 'forget' about missed events. We depend on
>>>> knowing which events were missed in order to be able to reissue them.
>>>
>>>> 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).
>>>
>>> Sorry, but I dont see how edma_stop() is coming into picture in the race
>>> you describe?
>>
>> In edma_callback function, for the case of DMA_COMPLETE (Transfer
>> completion interrupt), edma_stop() is called when all sets have been
>> processed. This had the effect of clearing the EMR.
> 
> Ah, thanks. I was missing the fact that the race comes into picture only
> when using the DMA engine driver. I guess that should be mentioned
> somewhere since it is not immediately obvious.
> 
> The patch looks good to me. So if you respin just this one with some
> updated explanation based on what you wrote below, I will take it.

Sure I'll do that. Also the trigger_channel patch, will you be taking
that one too? I can send these 2 in a series as they touch
arch/arm/common/edma.c

Thanks,

-Joel



> 
> Thanks,
> Sekhar
> 
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