Re: [PATCH 03/11] dt-bindings: Add RISC-V IOMMU bindings

[Zong Li]

On Wed, Jul 26, 2023 at 11:21 AM Baolu Lu <baolu.lu@xxxxxxxxxxxxxxx> wrote:
>
> On 2023/7/24 21:23, Zong Li wrote:
> >>>>> In RISC-V IOMMU, certain devices can be set to bypass mode when the
> >>>>> IOMMU is in translation mode. To identify the devices that require
> >>>>> bypass mode by default, does it be sensible to add a property to
> >>>>> indicate this behavior?
> >>>> Bypass mode for a device is a property of that device (similar to dma-coherent)
> >>>> and not of the IOMMU. Other architectures (ARM and x86) never added such
> >>>> a device property for bypass mode so I guess it is NOT ADVISABLE to do it.
> >>>>
> >>>> If this is REALLY required then we can do something similar to the QCOM
> >>>> SMMU driver where they have a whitelist of devices which are allowed to
> >>>> be in bypass mode (i.e. IOMMU_DOMAIN_IDENTITY) based their device
> >>>> compatible string and any device outside this whitelist is blocked by default.
> >>>>
> >>> I have considered that adding the property of bypass mode to that
> >>> device would be more appropriate. However, if we want to define this
> >>> property for the device, it might need to go through the generic IOMMU
> >>> dt-bindings, but I'm not sure if other IOMMU devices need this. I am
> >>> bringing up this topic here because I would like to explore if there
> >>> are any solutions on the IOMMU side, such as a property that indicates
> >>> the phandle of devices wishing to set bypass mode, somewhat similar to
> >>> the whitelist you mentioned earlier. Do you think we should address
> >>> this? After all, this is a case of RISC-V IOMMU supported.
> >> Bypass mode is a common feature across IOMMUs. Other IOMMUs don't
> >> have a special property for bypass mode at device-level or at IOMMU level,
> >> which clearly indicates that defining a RISC-V specific property is not the
> >> right way to go.
> >>
> >> The real question is how do we set IOMMU_DOMAIN_IDENTITY (i.e.
> >> bypass/identity domain) as the default domain for certain devices ?
> >>
> >> One possible option is to implement def_domain_type() IOMMU operation
> >> for RISC-V IOMMU which will return IOMMU_DOMAIN_IDENTITY for
> >> certain devices based on compatible string matching (i.e. whitelist of
> >> devices). As an example, refer qcom_smmu_def_domain_type()
> >> of drivers/iommu/arm/arm-smmu/arm-smmu-qcom.c
> >>
> > That is indeed one way to approach it, and we can modify the
> > compatible string when we want to change the mode. However, it would
> > be preferable to explore a more flexible approach to achieve this
> > goal. By doing so, we can avoid hard coding anything in the driver or
> > having to rebuild the kernel  whenever we want to change the mode for
> > certain devices. While I have considered extending a cell in the
> > 'iommus' property to indicate a device's desire to set bypass mode, it
> > doesn't comply with the iommu documentation and could lead to
> > ambiguous definitions.
>
> Hard coding the matching strings in the iommu driver is definitely not a
> preferable way. A feasible solution from current code's point of view is
> that platform opt-in the device's special requirements through DT or
> ACPI. And in the def_domain_type callback, let the iommu core know that,
> hence it can allocate a right type of domain for the device.
>
> Thoughts?
>

It would be nice if we can deal with it at this time. As we discussed
earlier, we might need to consider how to indicate that, such as
putting a property in device side or iommu side, and whether we need
to define it in generic dt-binding instead of RISC-V specific
dt-binding.

> Best regards,
> baolu