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

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Date:   Mon, 7 Aug 2017 13:57:35 +0530
From:   Vivek Gautam <vivek.gautam@...eaurora.org>
To:     Rob Clark <robdclark@...il.com>,
        Robin Murphy <robin.murphy@....com>
Cc:     Sricharan R <sricharan@...eaurora.org>,
        Stephen Boyd <sboyd@...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>,
        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 5:20 PM, Rob Clark <robdclark@...il.com> wrote:
> On Thu, Jul 13, 2017 at 1:35 AM, Sricharan R <sricharan@...eaurora.org> wrote:
>> Hi Vivek,
>>
>> On 7/13/2017 10:43 AM, 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?
>>
>>  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).

I would like to understand whether there is a situation where an unmap is
called in atomic context without an enabled master?

Let's say we have the case where all the unmap calls in atomic context happen
only from the master's context (in which case the device link should
take care of
the pm state of smmu), and the only unmap that happen in non-atomic context
is the one with master disabled. In such a case doesn it make sense to
distinguish
the atomic/non-atomic context and add pm_runtime_get_sync()/put_sync() only
for the non-atomic context since that would be the one with master disabled.


Thanks
Vivek

> 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().
>
> BR,
> -R
> --
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