From: nwatters@codeaurora.org (nwatters at codeaurora.org)
To: linux-arm-kernel@lists.infradead.org
Subject: [PATCH v2] iommu/arm-smmu-v3: limit use of 2-level stream tables
Date: Thu, 14 Jul 2016 13:33:50 -0400 [thread overview]
Message-ID: <103772215d30a7c862f66200bbcc16ea@codeaurora.org> (raw)
In-Reply-To: <20160714133107.GK29455@arm.com>
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.
prev parent reply other threads:[~2016-07-14 17:33 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-07-11 18:00 [PATCH] iommu/arm-smmu-v3: limit use of 2-level stream tables Nate Watterson
2016-07-12 9:36 ` Robin Murphy
2016-07-12 18:19 ` [PATCH v2] " Nate Watterson
2016-07-14 13:31 ` Will Deacon
2016-07-14 17:33 ` nwatters at codeaurora.org [this message]
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