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

[Robin Murphy]

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@xxxxxxxxx/

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.