Re: [PATCH 1/1] iommu/arm-smmu: Add support to use Last level cache

[Vivek Gautam]

Hi Will,


On Wed, Jun 27, 2018 at 10:07 PM, Will Deacon <will.deacon@xxxxxxx> wrote:
> Hi Vivek,
>
> On Tue, Jun 19, 2018 at 02:04:44PM +0530, Vivek Gautam wrote:
>> On Fri, Jun 15, 2018 at 10:22 PM, Will Deacon <will.deacon@xxxxxxx> wrote:
>> > On Fri, Jun 15, 2018 at 04:23:29PM +0530, Vivek Gautam wrote:
>> >> Qualcomm SoCs have an additional level of cache called as
>> >> System cache or Last level cache[1]. This cache sits right
>> >> before the DDR, and is tightly coupled with the memory
>> >> controller.
>> >> The cache is available to all the clients present in the
>> >> SoC system. The clients request their slices from this system
>> >> cache, make it active, and can then start using it. For these
>> >> clients with smmu, to start using the system cache for
>> >> dma buffers and related page tables [2], few of the memory
>> >> attributes need to be set accordingly.
>> >> This change makes the related memory Outer-Shareable, and
>> >> updates the MAIR with necessary protection.
>> >>
>> >> The MAIR attribute requirements are:
>> >>     Inner Cacheablity = 0
>> >>     Outer Cacheablity = 1, Write-Back Write Allocate
>> >>     Outer Shareablity = 1
>> >
>> > Hmm, so is this cache coherent with the CPU or not?
>>
>> Thanks for reviewing.
>> Yes, this LLC is cache coherent with CPU, so we mark for Outer-cacheable.
>> The different masters such as GPU as able to allocated and activate a slice
>> in this Last Level Cache.
>
> What I mean is, for example, if the CPU writes some data using Normal, Inner
> Shareable, Inner/Outer Cacheable, Inner/Outer Write-back, Non-transient
> Read/Write-allocate and a device reads that data using your MAIR encoding
> above, is the device guaranteed to see the CPU writes after the CPU has
> executed a DSB instruction?
>
> I don't think so, because the ARM ARM would say that there's a mismatch on
> the Inner Cacheability attribute.
>
>> > Why don't normal
>> > non-cacheable mappings allocated in the LLC by default?
>>
>> Sorry, I couldn't fully understand your question here.
>> Few of the masters on qcom socs are not io-coherent, so for them
>> the IC has to be marked as 0.
>
> By IC you mean Inner Cacheability? In your MAIR encoding above, it is zero
> so I don't understand the problem. What goes wrong if non-coherent devices
> use your MAIR encoding for their DMA buffers?
>
>> But they are able to use the LLC with OC marked as 1.
>
> The issue here is that whatever attributes we put in the SMMU need to align
> with the attributes used by the CPU in order to avoid introducing mismatched
> aliases. Currently, we support three types of mapping in the SMMU:
>
> 1. DMA non-coherent (e.g. "dma-coherent" is not set on the device)
>         Normal, Inner Shareable, Inner/Outer Non-Cacheable
>
> 2. DMA coherent (e.g. "dma-coherent" is set on the device) [IOMMU_CACHE]
>         Normal, Inner Shareable, Inner/Outer Cacheable, Inner/Outer
>         Write-back, Non-transient Read/Write-allocate
>
> 3. MMIO (e.g. MSI doorbell) [IOMMU_MMIO]
>         Device-nGnRE (Outer Shareable)
>
> So either you override one of these types (I was suggesting (1)) or you need
> to create a new memory type, along with the infrastructure for it to be
> recognised on a per-device basis and used by the DMA API so that we don't
> get mismatched aliases on the CPU.

My apologies for delay in responding to this thread.
I have been digging and getting in touch with internal tech teams
to get more information on this. I will update as soon as I have enough
details.
Thanks.

Best regards
Vivek

>
> Will
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