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

Vivek Gautam vivek.gautam at codeaurora.org
Thu Sep 20 04:41:53 PDT 2018

Hi Will,

On Wed, Jun 27, 2018 at 10:07 PM Will Deacon <will.deacon at arm.com> 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 at arm.com> 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?

No, these MAIR configurations don't guarantee that devices will have
coherent view
of what CPU writes. Not all devices can snoop into CPU caches (only IO-Coherent
devices can).
So a normal cached memory configuration in CPU MMU tables, and SMMU page tables
is valid only for few devices that are IO-coherent.

Moreover, CPU can lookup in system cache, and so do all devices;
allocation will depend on h/w configurations and memory attributes.
So anything that CPU caches in system cache will be coherently visible
to devices.

> 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.

Not really, right?
Devices can use Inner non-Cacheable, Outer-cacheable (IC=0, OC=1) to allocate
into the system cache (as these devices don't want to allocate in
their inner caches),
and the CPU will have a coherent view of these buffers/page-tables.
This should be
a normal cached non-IO-Coherent memory.

But anything that CPU writes using Normal, Inner Shareable,
Inner/Outer Cacheable,
Inner/Outer Write-back, Non-transient Read/Write-allocate, may not be visible
to the device.

Also added Jordan, and Pratik to this thread.

Thanks & Regards

> 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.
> Will
> _______________________________________________
> iommu mailing list
> iommu at lists.linux-foundation.org
> https://lists.linuxfoundation.org/mailman/listinfo/iommu

QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a member
of Code Aurora Forum, hosted by The Linux Foundation

More information about the linux-arm-kernel mailing list