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