linux-kernel - RE: [PATCH] iommu/io-pgtable-arm: Optimize partial walk flush for large scatter-gather list

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Date:   Fri, 11 Jun 2021 16:49:54 +0000
From:   Krishna Reddy <vdumpa@...dia.com>
To:     Sai Prakash Ranjan <saiprakash.ranjan@...eaurora.org>
CC:     Robin Murphy <robin.murphy@....com>,
        "linux-arm-msm@...r.kernel.org" <linux-arm-msm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        Will Deacon <will@...nel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        Thierry Reding <treding@...dia.com>
Subject: RE: [PATCH] iommu/io-pgtable-arm: Optimize partial walk flush for
 large scatter-gather list

Hi Sai,
> >> > No, the unmap latency is not just in some test case written, the
> >> > issue is very real and we have workloads where camera is reporting
> >> > frame drops because of this unmap latency in the order of 100s of
> milliseconds.

> Not exactly, this issue is not specific to camera. If you look at the numbers in the
> commit text, even for the test device its the same observation. It depends on
> the buffer size we are unmapping which affects the number of TLBIs issue. I am
> not aware of any such HW side bw issues for camera specifically on QCOM
> devices.

It is clear that reducing number of TLBIs  reduces the umap API latency. But, It is
at the expense of throwing away valid tlb entries. 
Quantifying the impact of arbitrary invalidation of valid tlb entries at context level is not straight forward and
use case dependent. The side-effects might be rare or won't be known until they are noticed.
Can you provide more details on How the unmap latency is causing camera to drop frames?
Is unmap performed in the perf path?
If unmap is queued and performed on a back ground thread, would it resolve the frame drops?

-KR