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 15:05:24 +0530
From:	Sekhar Nori <nsekhar@...com>
To:	Joel Fernandes <joelf@...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 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.

Thanks,
Sekhar

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