linux-kernel - Re: [PATCH 1/6] iommu/qcom: Use the asid read from device-tree if specified

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Message-ID: <20220531161910.GE25502@willie-the-truck>
Date:   Tue, 31 May 2022 17:19:11 +0100
From:   Will Deacon <will@...nel.org>
To:     Rob Clark <robdclark@...il.com>
Cc:     Konrad Dybcio <konrad.dybcio@...ainline.org>,
        ~postmarketos/upstreaming@...ts.sr.ht,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        "moderated list:ARM/FREESCALE IMX / MXC ARM ARCHITECTURE" 
        <linux-arm-kernel@...ts.infradead.org>,
        "list@....net:IOMMU DRIVERS <iommu@...ts.linux-foundation.org>, Joerg
        Roedel <joro@...tes.org>," <iommu@...ts.linux-foundation.org>,
        Martin Botka <martin.botka@...ainline.org>,
        AngeloGioacchino Del Regno 
        <angelogioacchino.delregno@...ainline.org>,
        Marijn Suijten <marijn.suijten@...ainline.org>,
        jamipkettunen@...ainline.org, Andy Gross <agross@...nel.org>,
        Joerg Roedel <joro@...tes.org>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Robin Murphy <robin.murphy@....com>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/6] iommu/qcom: Use the asid read from device-tree if
 specified

On Tue, May 31, 2022 at 09:15:22AM -0700, Rob Clark wrote:
> On Tue, May 31, 2022 at 8:46 AM Will Deacon <will@...nel.org> wrote:
> >
> > On Fri, May 27, 2022 at 11:28:56PM +0200, Konrad Dybcio wrote:
> > > From: AngeloGioacchino Del Regno <angelogioacchino.delregno@...ainline.org>
> > >
> > > As specified in this driver, the context banks are 0x1000 apart.
> > > Problem is that sometimes the context number (our asid) does not
> > > match this logic and we end up using the wrong one: this starts
> > > being a problem in the case that we need to send TZ commands
> > > to do anything on a specific context.
> >
> > I don't understand this. The ASID is a software construct, so it shouldn't
> > matter what we use. If it does matter, then please can you explain why? The
> > fact that the context banks are 0x1000 apart seems unrelated.
> 
> I think the connection is that mapping from ctx bank to ASID is 1:1

But in what sense? How is the ASID used beyond a tag in the TLB? The commit
message hints at "TZ commands" being a problem.

I'm not doubting that this is needed to make the thing work, I just don't
understand why.

Will