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From:   "Krcka, Tomas" <krckatom@amazon.de>
To:     Will Deacon <will@kernel.org>
CC:     "Krcka, Tomas" <krckatom@amazon.de>,
        "linux-arm-kernel@lists.infradead.org" 
        <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" <iommu@lists.linux.dev>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] iommu/arm-smmu-v3: Fix event queue overflow
 acknowledgment
Thread-Topic: [PATCH] iommu/arm-smmu-v3: Fix event queue overflow
 acknowledgment
Thread-Index: AQHZYjsUJTyLbHuMvUep4ekJH0WoLQ==
Date:   Wed, 29 Mar 2023 12:36:26 +0000
Message-ID: <BD6629C9-7E59-43B1-BE58-F6799E0FBC5E@amazon.com>
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> On 28. Mar 2023, at 13:56, Will Deacon <will@kernel.org> wrote:
> =

> =

> On Tue, Mar 28, 2023 at 07:13:52AM +0000, Krcka, Tomas wrote:
>> =

>>> 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 toggli=
ng
>>>> flag OVACKFLG in the CONS register, otherwise the overflow condition is
>>>> still active (OVFLG !=3D 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/iom=
mu/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 =3D 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 thi=
nk
>>> 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 w=
rite back
>> every time.
> =

> Sure, feel free to update both.
> =


OK, I sent it as new patch: https://lore.kernel.org/lkml/20230329123420.346=
41-1-tomas.krcka@gmail.com


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