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

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Date:   Tue, 28 Mar 2023 12:56:13 +0100
From:   Will Deacon <will@...nel.org>
To:     "Krcka, Tomas" <krckatom@...zon.de>
Cc:     "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        Robin Murphy <robin.murphy@....com>,
        Joerg Roedel <joro@...tes.org>,
        Lu Baolu <baolu.lu@...ux.intel.com>,
        Shameer Kolothum <shameerali.kolothum.thodi@...wei.com>,
        "iommu@...ts.linux.dev" <iommu@...ts.linux.dev>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] iommu/arm-smmu-v3: Fix event queue overflow
 acknowledgment

On Tue, Mar 28, 2023 at 07:13:52AM +0000, Krcka, Tomas wrote:
> 
> > On 27. Mar 2023, at 14:12, Will Deacon <will@...nel.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@...zon.de>
> >> Suggested-by: KarimAllah Ahmed <karahmed@...zon.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.

Sure, feel free to update both.

Will