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Date:   Mon, 27 Mar 2023 13:12:34 +0100
From:   Will Deacon <will@kernel.org>
To:     Tomas Krcka <krckatom@amazon.de>
Cc:     linux-arm-kernel@lists.infradead.org,
        Robin Murphy <robin.murphy@arm.com>,
        Joerg Roedel <joro@8bytes.org>,
        Lu Baolu <baolu.lu@linux.intel.com>,
        Shameer Kolothum <shameerali.kolothum.thodi@huawei.com>,
        iommu@lists.linux.dev, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] iommu/arm-smmu-v3: Fix event queue overflow
 acknowledgment
Message-ID: <20230327121234.GA31342@willie-the-truck>
References: <20230308092048.71390-1-krckatom@amazon.de>
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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