linux-kernel - Re: [PATCH V4 3/6] iommu/arm-smmu: Invoke pm_runtime during probe, add/remove device

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Message-id: <4f2b6272-d938-767f-6ea3-0b515374d1a1@samsung.com>
Date:   Thu, 13 Jul 2017 14:23:54 +0200
From:   Marek Szyprowski <m.szyprowski@...sung.com>
To:     Rob Clark <robdclark@...il.com>
Cc:     Sricharan R <sricharan@...eaurora.org>,
        Vivek Gautam <vivek.gautam@...eaurora.org>,
        Stephen Boyd <sboyd@...eaurora.org>,
        Joerg Roedel <joro@...tes.org>,
        Robin Murphy <robin.murphy@....com>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Will Deacon <will.deacon@....com>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-clk <linux-clk@...r.kernel.org>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        Stanimir Varbanov <stanimir.varbanov@...aro.org>,
        Archit Taneja <architt@...eaurora.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH V4 3/6] iommu/arm-smmu: Invoke pm_runtime during probe,
 add/remove device

Hi Rob,

On 2017-07-13 14:10, Rob Clark wrote:
> On Thu, Jul 13, 2017 at 8:02 AM, Marek Szyprowski
> <m.szyprowski@...sung.com> wrote:
>> On 2017-07-13 13:50, Rob Clark wrote:
>>> On Thu, Jul 13, 2017 at 1:35 AM, Sricharan R <sricharan@...eaurora.org>
>>> wrote:
>>>> On 7/13/2017 10:43 AM, Vivek Gautam wrote:
>>>>> On 07/13/2017 04:24 AM, Stephen Boyd wrote:
>>>>>> On 07/06, Vivek Gautam wrote:
>>>>>>> @@ -1231,12 +1237,18 @@ static int arm_smmu_map(struct iommu_domain
>>>>>>> *domain, unsigned long iova,
>>>>>>>     static size_t arm_smmu_unmap(struct iommu_domain *domain, unsigned
>>>>>>> long iova,
>>>>>>>                      size_t size)
>>>>>>>     {
>>>>>>> -    struct io_pgtable_ops *ops = to_smmu_domain(domain)->pgtbl_ops;
>>>>>>> +    struct arm_smmu_domain *smmu_domain = to_smmu_domain(domain);
>>>>>>> +    struct io_pgtable_ops *ops = smmu_domain->pgtbl_ops;
>>>>>>> +    size_t ret;
>>>>>>>           if (!ops)
>>>>>>>             return 0;
>>>>>>>     -    return ops->unmap(ops, iova, size);
>>>>>>> +    pm_runtime_get_sync(smmu_domain->smmu->dev);
>>>>>> Can these map/unmap ops be called from an atomic context? I seem
>>>>>> to recall that being a problem before.
>>>>> That's something which was dropped in the following patch merged in
>>>>> master:
>>>>> 523d7423e21b iommu/arm-smmu: Remove io-pgtable spinlock
>>>>>
>>>>> Looks like we don't  need locks here anymore?
>>>>    Apart from the locking, wonder why a explicit pm_runtime is needed
>>>>    from unmap. Somehow looks like some path in the master using that
>>>>    should have enabled the pm ?
>>>>
>>> Yes, there are a bunch of scenarios where unmap can happen with
>>> disabled master (but not in atomic context).  On the gpu side we
>>> opportunistically keep a buffer mapping until the buffer is freed
>>> (which can happen after gpu is disabled).  Likewise, v4l2 won't unmap
>>> an exported dmabuf while some other driver holds a reference to it
>>> (which can be dropped when the v4l2 device is suspended).
>>>
>>> Since unmap triggers tbl flush which touches iommu regs, the iommu
>>> driver *definitely* needs a pm_runtime_get_sync().
>>
>> Afair unmap might be called from atomic context as well, for example as
>> a result of dma_unmap_page(). In exynos IOMMU I simply check the runtime
>> PM state of IOMMU device. TLB flush is performed only when IOMMU is in
>> active
>> state. If it is suspended, I assume that the IOMMU controller's context
>> is already lost and its respective power domain might be already turned off,
>> so there is no point in touching IOMMU registers.
>>
> that seems like an interesting approach.. although I wonder if there
> can be some race w/ new device memory access once clks are enabled
> before tlb flush completes?  That would be rather bad, since this
> approach is letting the backing pages of memory be freed before tlb
> flush.

Exynos IOMMU has spinlock for ensuring that there is no race between PM 
runtime
suspend and unmap/tlb flush.

Best regards
-- 
Marek Szyprowski, PhD
Samsung R&D Institute Poland