Re: [PATCH v2] iommu/arm-smmu-v3: limit use of 2-level stream tables

[nwatters]

On 2016-07-14 09:31, Will Deacon wrote:
> On Tue, Jul 12, 2016 at 02:19:20PM -0400, Nate Watterson wrote:
> > In the current arm-smmu-v3 driver, all smmus that support 2-level
> > stream tables are being forced to use them. This is suboptimal for
> > smmus that support fewer stream id bits than would fill in a single
> > second level table. This patch limits the use of 2-level tables to
> > smmus that both support the feature and whose first level table can
> > possibly contain more than a single entry.
>
> Just to be clear, what exactly are you seeing as being suboptimal here?
> Is it the memory wastage from overallocating the L2 table, or something
> more?

Disregarding the config cache, fetching an STE when 2-level tables are
being used will require the hw to perform more memory accesses than it
would have to with a linear table since the L1 descriptor must also be
fetched. Presumably this is why the spec states, "ARM recommends that
a more efficient linear table is used instead of programming SPLIT >
LOG2SIZE".

My understanding is that the only benefit to using 2-level tables is
that it can save space when stream ids are sparsely distributed. Are
there any other compelling reasons to use them?

> if it's just the memory allocation, I'd sooner restrict the span field
> in the L1 desc.

Although I am not especially concerned about the memory allocation, even
if the span was reduced, we would still be wasting a page for the L1
table unless L1 and L2 tables were allocated in a single 
dmam_alloc_coherent
call.

> Will

Nate

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