Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > linux.kernel > #1443632

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

Show all headers | View raw

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

-- 
Qualcomm Datacenter Technologies, Inc. on behalf of Qualcomm 
Technologies, Inc.
Qualcomm Technologies, Inc. is a member of the Code Aurora Forum, a 
Linux Foundation Collaborative Project.

Back to linux.kernel | Previous | Next — Previous in thread | Find similar | Unroll thread

Thread

[PATCH] iommu/arm-smmu-v3: limit use of 2-level stream tables Nate Watterson <nwatters@codeaurora.org> - 2016-07-11 20:10 +0200
  Re: [PATCH] iommu/arm-smmu-v3: limit use of 2-level stream tables Robin Murphy <robin.murphy@arm.com> - 2016-07-12 11:40 +0200
  [PATCH v2] iommu/arm-smmu-v3: limit use of 2-level stream tables Nate Watterson <nwatters@codeaurora.org> - 2016-07-12 20:30 +0200
    Re: [PATCH v2] iommu/arm-smmu-v3: limit use of 2-level stream tables Will Deacon <will.deacon@arm.com> - 2016-07-14 15:40 +0200
      Re: [PATCH v2] iommu/arm-smmu-v3: limit use of 2-level stream tables nwatters@codeaurora.org - 2016-07-14 19:40 +0200

csiph-web