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

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Date:   Thu, 13 Jul 2017 07:53:42 -0400
From:   Rob Clark <robdclark@...il.com>
To:     Robin Murphy <robin.murphy@....com>
Cc:     Stephen Boyd <sboyd@...eaurora.org>,
        Vivek Gautam <vivek.gautam@...eaurora.org>,
        Joerg Roedel <joro@...tes.org>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Will Deacon <will.deacon@....com>,
        Marek Szyprowski <m.szyprowski@...sung.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>,
        Sricharan R <sricharan@...eaurora.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

On Thu, Jul 13, 2017 at 5:50 AM, Robin Murphy <robin.murphy@....com> wrote:
> On 13/07/17 07:48, Stephen Boyd wrote:
>> On 07/13, Vivek Gautam wrote:
>>> Hi Stephen,
>>>
>>>
>>> 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?
>>>
>>
>> While removing the spinlock around the map/unmap path may be one
>> thing, I'm not sure that's all of them. Is there a path from an
>> atomic DMA allocation (GFP_ATOMIC sort of thing) mapped into an
>> IOMMU for a device that can eventually get down to here and
>> attempt to turn a clk on?
>
> Yes, in the DMA path map/unmap will frequently be called from IRQ
> handlers (think e.g. network packets). The whole point of removing the
> lock was to allow multiple maps/unmaps to execute in parallel (since we
> know they will be safely operating on different areas of the pagetable).
> AFAICS this change is going to largely reintroduce that bottleneck via
> dev->power_lock, which is anything but what we want :(
>

Maybe __pm_runtime_resume() needs some sort of fast-path if already
enabled?  Or otherwise we need some sort of flag to tell the iommu
that it cannot rely on the unmapping device to be resumed?

BR,
-R