linux-kernel - Re: [PATCH 2/6] arm64: dts: qcom: sm8250: add apps

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Message-ID: <2a35f3b85d8311fb4298aaea82236967@codeaurora.org>
Date:   Mon, 25 May 2020 16:24:54 +0530
From:   Sai Prakash Ranjan <saiprakash.ranjan@...eaurora.org>
To:     Jonathan Marek <jonathan@...ek.ca>
Cc:     linux-arm-msm@...r.kernel.org, Andy Gross <agross@...nel.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Rob Herring <robh+dt@...nel.org>, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-arm-msm-owner@...r.kernel.org
Subject: Re: [PATCH 2/6] arm64: dts: qcom: sm8250: add apps_smmu node

On 2020-05-25 15:39, Jonathan Marek wrote:
> Hi,
> 
> On 5/25/20 5:42 AM, Sai Prakash Ranjan wrote:
>> Hi Jonathan,
>> 
>> On 2020-05-24 08:08, Jonathan Marek wrote:
>>> Add the apps_smmu node for sm8250. Note that adding the iommus field 
>>> for
>>> UFS is required because initializing the iommu removes the bypass 
>>> mapping
>>> that created by the bootloader.
>>> 
>> 
>> This statement doesn't seem right, you can just say since the bypass 
>> is disabled
>> by default now, we need to add this property to enable translation and 
>> avoid global faults.
>> 
> 
> If I use this patch [1] then the UFS iommu property isn't needed. In
> downstream, the identity (bypass?) stream mapping is inherited from
> the bootloader, and UFS is used without any iommu property. Setting
> ARM_SMMU_DISABLE_BYPASS_BY_DEFAULT=n doesn't make it work on its own
> (without the UFS iommu property), so there's more to it than just
> "bypass is disabled by default now".
> 
> https://patchwork.kernel.org/patch/11310757/
> 

"iommus" property is not about inheriting stream mapping from 
bootloader,
it is used to enable SMMU address translation for the corresponding
master when specified. So when you have disabled bypass, i.e.,
ARM_SMMU_DISABLE_BYPASS_BY_DEFAULT=y or via cmdline 
"arm-smmu.disable_bypass=1"
and iommus property with SID and mask is not specified, then it will 
result
in SMMU global faults.

Downstream has bypass enabled(ARM_SMMU_DISABLE_BYPASS_BY_DEFAULT=n),so 
you
won't see any global faults if you do not have iommus property.

Patch in your link is for display because of the usecase for splash 
screen
on android and some other devices where the bootloader will configure 
SMMU,
it has not yet merged and not likely to get merged in the current state.

So yes "there is *not* much more to it than bypass is disabled by 
default now"
and you have to specify "iommus" for the master devices or you should 
enable bypass,
i.e., ARM_SMMU_DISABLE_BYPASS_BY_DEFAULT=n or arm-smmu.disable_bypass=n

Try without the patch in the link and without iommus for UFS and
ARM_SMMU_DISABLE_BYPASS_BY_DEFAULT=y and you will see.

-Sai
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