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

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Date:   Fri, 14 Jul 2017 13:42:13 -0400
From:   Rob Clark <robdclark@...il.com>
To:     Will Deacon <will.deacon@....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>,
        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 Fri, Jul 14, 2017 at 1:07 PM, Will Deacon <will.deacon@....com> wrote:
> On Thu, Jul 13, 2017 at 10:55:10AM -0400, Rob Clark wrote:
>> On Thu, Jul 13, 2017 at 9:53 AM, Sricharan R <sricharan@...eaurora.org> wrote:
>> > Hi,
>> >
>> > On 7/13/2017 5:20 PM, Rob Clark 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).  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().
>> >
>> >  Ok, with that being the case, there are two things here,
>> >
>> >  1) If the device links are still intact at these places where unmap is called,
>> >     then pm_runtime from the master would setup the all the clocks. That would
>> >     avoid reintroducing the locking indirectly here.
>> >
>> >  2) If not, then doing it here is the only way. But for both cases, since
>> >     the unmap can be called from atomic context, resume handler here should
>> >     avoid doing clk_prepare_enable , instead move the clk_prepare to the init.
>> >
>>
>> I do kinda like the approach Marek suggested.. of deferring the tlb
>> flush until resume.  I'm wondering if we could combine that with
>> putting the mmu in a stalled state when we suspend (and not resume the
>> mmu until after the pending tlb flush)?
>
> I'm not sure that a stalled state is what we're after here, because we need
> to take care to prevent any table walks if we've freed the underlying pages.
> What we could try to do is disable the SMMU (put into global bypass) and
> invalidate the TLB when performing a suspend operation, then we just ignore
> invalidation whilst the clocks are stopped and, on resume, enable the SMMU
> again.

wouldn't stalled just block any memory transactions by device(s) using
the context bank?  Putting it in bypass isn't really a good thing if
there is any chance the device can sneak in a memory access before
we've taking it back out of bypass (ie. makes gpu a giant userspace
controlled root hole).

BR,
-R

> That said, I don't think we can tolerate suspend/resume racing with
> map/unmap, and it's not clear to me how we avoid that without penalising
> the fastpath.
>
> Will