linux-kernel - Re: [PATCH] iomm/arm-smmu: Add stall implementation hook

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Message-ID: <20200520125916.GE25815@willie-the-truck>
Date:   Wed, 20 May 2020 13:59:17 +0100
From:   Will Deacon <will@...nel.org>
To:     Sai Prakash Ranjan <saiprakash.ranjan@...eaurora.org>
Cc:     Rob Clark <robdclark@...il.com>,
        Robin Murphy <robin.murphy@....com>,
        Joerg Roedel <joro@...tes.org>,
        "list@....net:IOMMU DRIVERS , Joerg Roedel <joro@...tes.org>," 
        <iommu@...ts.linux-foundation.org>,
        "moderated list:ARM/FREESCALE IMX / MXC ARM ARCHITECTURE" 
        <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>
Subject: Re: [PATCH] iomm/arm-smmu: Add stall implementation hook

On Wed, May 20, 2020 at 03:02:45PM +0530, Sai Prakash Ranjan wrote:
> On 2020-05-19 20:41, Rob Clark wrote:
> > On Tue, May 19, 2020 at 2:26 AM Sai Prakash Ranjan
> > <saiprakash.ranjan@...eaurora.org> wrote:
> > > On 2020-05-18 21:15, Will Deacon wrote:
> > > > So this sounds like an erratum to me, and I'm happy to set HUPCF if we
> > > > detect the broken implementation. However, it will need an entry in
> > > > Documentation/arm64/silicon-errata.rst and a decent comment in the
> > > > driver
> > > > to explain what we're doing and why.
> > > >
> > > 
> > > AFAIK there is no erratum documented internally for this behaviour and
> > > this
> > > exists from MSM8996 SoC time and errata usually don't survive this
> > > long
> > > across generation of SoCs and there is no point for us in disguising
> > > it.
> > 
> > possibly longer, qcom_iommu sets CFCFG..
> > 
> 
> Oh right, I was still in college when those SoCs were released ;)
> 
> > > Is it OK if we clearly mention it as the "design limitation" or some
> > > other
> > > term which we can agree upon along with the description which Rob and
> > > Jordan
> > > provided for setting HUPCF in the driver when we add the set_hupcf
> > > callback?
> > 
> > I'm not too picky on what we call it, but afaict it has been this way
> > since the beginning of time, rather than specific to a certain SoC or
> > generation of SoCs.  So it doesn't seem like the hw designers consider
> > it a bug.
> > 
> > (I'm not sure what the expected behavior is.. nor if any other SMMU
> > implementation encounters this sort of situation..)
> 
> Yes, that was my point as well that its probably not counted as a bug
> by the hw designers. So I'm going to post setting HUPCF on QCOM
> implementation with clear comments based on yours and Jordan's description
> of this problem, but I wanted to have a way to set this only for GPU context
> bank and not GMU as Jordan mentioned earlier that GMU doesnt need HUPCF set.
> I was checking as to how do we map cb to device, if it was possible then we
> can have
> a compatibility thing like we did for identity mapping. Any ideas Robin?

Right, see my reply over at:

https://lore.kernel.org/r/20200520125700.GD25815@willie-the-truck

Hopefully something like that can be made to work.

Will