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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