[PATCH v3 02/12] ARM: edma: Don't clear EMR of channel in edma_stop

[Joel Fernandes]

On Sun, Aug 11, 2013 at 11:25 PM, Sekhar Nori <nsekhar at ti.com> wrote:
> On 8/8/2013 5:19 PM, Sekhar Nori wrote:
>> 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 at ti.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()
>
> Forgot to ask, should this be tagged for stable? IOW, how serious is
> this race in current kernel (without the entire series applied)? I have
> never observed it myself - so please provide details how easy/difficult
> it is to hit this condition.

The race was uncovered by recent EDMA patch series, So this patch can
go in for next kernel release as such, I am not aware of any other DMA
user that maybe uncovering the race condition.

Thanks,

-Joel