linux-kernel - Re: [PATCH 0/3] Request direct mapping for modem firmware subdevice

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Message-ID: <a50040a9-54fe-f682-dd7e-b2991b48d633@arm.com>
Date:   Tue, 10 Mar 2020 16:44:12 +0000
From:   Robin Murphy <robin.murphy@....com>
To:     Joerg Roedel <joro@...tes.org>, Sibi Sankar <sibis@...eaurora.org>
Cc:     ohad@...ery.com, devicetree@...r.kernel.org,
        linux-kernel-owner@...r.kernel.org, linux-arm-msm@...r.kernel.org,
        linux-remoteproc@...r.kernel.org, linux-kernel@...r.kernel.org,
        bjorn.andersson@...aro.org, iommu@...ts.linux-foundation.org,
        robh+dt@...nel.org, agross@...nel.org
Subject: Re: [PATCH 0/3] Request direct mapping for modem firmware subdevice

On 10/03/2020 4:23 pm, Joerg Roedel wrote:
> On Tue, Mar 10, 2020 at 07:30:50PM +0530, Sibi Sankar wrote:
>> The accesses are initiated by the firmware
>> and they access modem reserved regions.
>> However as explained in ^^ any accesses
>> outside the region will result in a violation
>> and is controlled through XPUs (protection units).
> 
> Okay, this sounds like a case for arm_smmu_get_resv_region(). It should
> return an entry for the reserved memory region the firmware needs to
> access, so that generic iommu can setup this mapping.
> 
> Note that it should return that entry only for your device, not for all
> devices. Maybe there is a property in DT or IORT you can set to
> transport this information into the arm-smmu driver.
> 
> This is pretty similar to RMRR mapping on the Intel VT-d IOMMU or
> Unity-mapped ranges in the AMD-Vi IOMMU.

Yup, a way to describe boot-time memory regions in IORT is in the 
process of being specced out; the first attempt at an equivalent for DT 
is here:

https://lore.kernel.org/linux-iommu/20191209150748.2471814-1-thierry.reding@gmail.com/

If that's not enough and the SMMU still needs to treat certain Stream 
IDs specially because they may be untranslatable (due to having direct 
access to memory as a side-channel), then that should be handled in the 
SoC-specific corner of the SMMU driver, not delegated to individual 
endpoint drivers.

Robin.