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

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Date:	Thu, 8 Aug 2013 17:19:39 +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 v3 02/12] ARM: edma: Don't clear EMR of channel in edma_stop

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()

Thanks,
Sekhar

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