Messages in this thread | | | From | Zong Li <> | Date | Wed, 26 Jul 2023 12:26:14 +0800 | Subject | Re: [PATCH 03/11] dt-bindings: Add RISC-V IOMMU bindings |
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On Wed, Jul 26, 2023 at 11:21 AM Baolu Lu <baolu.lu@linux.intel.com> 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
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