Re: [PATCH] iommu/arm-smmu-v3: Fix event queue overflow acknowledgment

["Krcka, Tomas" <krckatom@amazon.de>]

> On 27. Mar 2023, at 14:12, Will Deacon <will@kernel.org> wrote:
> 
> 
> On Wed, Mar 08, 2023 at 09:20:47AM +0000, Tomas Krcka wrote:
>> When an overflow occurs in the event queue, the SMMU toggles overflow
>> flag OVFLG in the PROD register.
>> The evtq thread is supposed to acknowledge the overflow flag by toggling
>> flag OVACKFLG in the CONS register, otherwise the overflow condition is
>> still active (OVFLG != OVACKFLG).
>> 
>> Currently the acknowledge register is toggled after clearing the event
>> queue but is never propagated to the hardware. It would be done next
>> time when executing evtq thread.
>> 
>> The SMMU still adds elements to the queue when the overflow condition is
>> active but any subsequent overflow information after clearing the event
>> queue will be lost.
>> 
>> This change keeps the SMMU in sync as it's expected by design.
>> 
>> Signed-off-by: Tomas Krcka <krckatom@amazon.de>
>> Suggested-by: KarimAllah Ahmed <karahmed@amazon.de>
>> ---
>> drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c | 1 +
>> 1 file changed, 1 insertion(+)
>> 
>> diff --git a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
>> index f2425b0f0cd6..acc1ff5ff69b 100644
>> --- a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
>> +++ b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
>> @@ -1579,6 +1579,7 @@ static irqreturn_t arm_smmu_evtq_thread(int irq, void *dev)
>>      /* Sync our overflow flag, as we believe we're up to speed */
>>      llq->cons = Q_OVF(llq->prod) | Q_WRP(llq, llq->cons) |
>>                  Q_IDX(llq, llq->cons);
>> +     queue_sync_cons_out(q);
>>      return IRQ_HANDLED;
>> }
> 
> I think I probably did mean to have something like this, but can we
> only do the actual h/w update if overflow has occurred? Otherwise I think
> we're pointlessly writing back the same value most of the time.
> 
> Will

Yes, we can, but then same applies for the priq as well, there we also write back
every time.

Tomas



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