Messages in this thread | | | Date | Wed, 31 Jul 2013 00:05:15 -0500 | From | Joel Fernandes <> | Subject | Re: [PATCH 7/9] ARM: edma: Don't clear EMR of channel in edma_stop |
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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.
This has 2 problems:
1. If error interrupt is also pending and TC interrupt clears the EMR.
Due to this the ARM will execute the error interrupt even though the EMR is clear. As a result, the following if condition in dma_ccerr_handler will be true and IRQ_NONE is returned.
if ((edma_read_array(ctlr, EDMA_EMR, 0) == 0) && (edma_read_array(ctlr, EDMA_EMR, 1) == 0) && (edma_read(ctlr, EDMA_QEMR) == 0) && (edma_read(ctlr, EDMA_CCERR) == 0)) return IRQ_NONE;
If this happens enough number of times, IRQ subsystem disables the interrupt thinking its spurious which creates serious problems.
2. If the above if statement condition is removed, then EMR is 0 so the callback function will not be called in dma_ccerr_handler thus the event is forgotten, never triggered manually or never sets missed flag of the channel.
So about the race: TC interrupt handler executing before the error interrupt handler can result in clearing the EMR and creates these problems.
>> The EMR is ultimately being cleared by the Error interrupt >> handler once it is handled so we don't have to do it in edma_stop. > > This, I agree with. edma_clean_channel() also there to re-initialize the > channel so doing it in edma_stop() certainly seems superfluous.
Sure.
Thanks,
-Joel
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