linux-kernel - Re: [PATCH] needsreview! arm64: dts: qcom: sdm845: Add missing MDSS reset

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Message-ID: <d2cd85cd-2df9-4738-a575-03b86f792067@ixit.cz>
Date: Tue, 13 Jan 2026 13:30:59 +0100
From: David Heidelberg <david@...t.cz>
To: Neil Armstrong <neil.armstrong@...aro.org>,
 Bjorn Andersson <andersson@...nel.org>,
 Konrad Dybcio <konradybcio@...nel.org>, Rob Herring <robh@...nel.org>,
 Krzysztof Kozlowski <krzk+dt@...nel.org>, Conor Dooley <conor+dt@...nel.org>
Cc: Petr Hodina <petr.hodina@...tonmail.com>, linux-arm-msm@...r.kernel.org,
 devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
 phone-devel@...r.kernel.org
Subject: Re: [PATCH] needsreview! arm64: dts: qcom: sdm845: Add missing MDSS
 reset

On 13/01/2026 10:48, Neil Armstrong wrote:
> On 1/12/26 13:33, David Heidelberg via B4 Relay wrote:
>> From: David Heidelberg <david@...t.cz>
>>
>> If the OS does not support recovering the state left by the
>> bootloader it needs a way to reset display hardware, so that it can
>> start from a clean state. Add a reference to the relevant reset.
>>
>> Signed-off-by: David Heidelberg <david@...t.cz>
>> ---
>> It efficiently fixes nothing for us (at least what we're aware), so I
>> assume the state left by bootloader is good enough
> 
> In fact it does even more, perhaps in your usecase you're not affected but
> it permits resetting the MDSS while switching the IOMMU from bypass to 
> translated.
> 
> In the current IOMMU implementation, there's no current way to keep the 
> IOMMU setup
> for an SID without a cut, leading to fatal IOMMU errors.
> 
> With the hypervisor enabled, some IOMMU entries are left to fallback in 
> bypass
> when switching, but in EL2 there's no fallback and the ARM SMMUv2 
> doesn't support
> mapping _before_ attaching to a device leading to:
> https://elixir.bootlin.com/linux/v6.18.4/source/drivers/iommu/iommu.c#L3046
>      /*
>       * Drivers are supposed to allow mappings to be installed in a domain
>       * before device attachment, but some don't. Hack around this 
> defect by
>       * trying again after attaching. If this happens it means the device
>       * will not continuously have the IOMMU_RESV_DIRECT map.
>       */
> 
> Sorry for the digression!
> 
> Neil
> 

Thank you, that's good to know! :)

David

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