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

[Zong Li <zong.li@sifive.com>]

On Mon, Jul 24, 2023 at 6:02 PM Anup Patel <apatel@ventanamicro.com> wrote:
>
> On Mon, Jul 24, 2023 at 1:33 PM Zong Li <zong.li@sifive.com> wrote:
> >
> > On Thu, Jul 20, 2023 at 3:35 AM Tomasz Jeznach <tjeznach@rivosinc.com> wrote:
> > >
> > > From: Anup Patel <apatel@ventanamicro.com>
> > >
> > > We add DT bindings document for RISC-V IOMMU platform and PCI devices
> > > defined by the RISC-V IOMMU specification.
> > >
> > > Signed-off-by: Anup Patel <apatel@ventanamicro.com>
> > > ---
> > >  .../bindings/iommu/riscv,iommu.yaml           | 146 ++++++++++++++++++
> > >  1 file changed, 146 insertions(+)
> > >  create mode 100644 Documentation/devicetree/bindings/iommu/riscv,iommu.yaml
> > >
> > > diff --git a/Documentation/devicetree/bindings/iommu/riscv,iommu.yaml b/Documentation/devicetree/bindings/iommu/riscv,iommu.yaml
> > > new file mode 100644
> > > index 000000000000..8a9aedb61768
> > > --- /dev/null
> > > +++ b/Documentation/devicetree/bindings/iommu/riscv,iommu.yaml
> > > @@ -0,0 +1,146 @@
> > > +# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
> > > +%YAML 1.2
> > > +---
> > > +$id: http://devicetree.org/schemas/iommu/riscv,iommu.yaml#
> > > +$schema: http://devicetree.org/meta-schemas/core.yaml#
> > > +
> > > +title: RISC-V IOMMU Implementation
> > > +
> > > +maintainers:
> > > +  - Tomasz Jeznach <tjeznach@rivosinc.com>
> > > +
> > > +description:
> > > +  The RISC-V IOMMU specificaiton defines an IOMMU for RISC-V platforms
> > > +  which can be a regular platform device or a PCI device connected to
> > > +  the host root port.
> > > +
> > > +  The RISC-V IOMMU provides two stage translation, device directory table,
> > > +  command queue and fault reporting as wired interrupt or MSIx event for
> > > +  both PCI and platform devices.
> > > +
> > > +  Visit https://github.com/riscv-non-isa/riscv-iommu for more details.
> > > +
> > > +properties:
> > > +  compatible:
> > > +    oneOf:
> > > +      - description: RISC-V IOMMU as a platform device
> > > +        items:
> > > +          - enum:
> > > +              - vendor,chip-iommu
> > > +          - const: riscv,iommu
> > > +
> > > +      - description: RISC-V IOMMU as a PCI device connected to root port
> > > +        items:
> > > +          - enum:
> > > +              - vendor,chip-pci-iommu
> > > +          - const: riscv,pci-iommu
> > > +
> > > +  reg:
> > > +    maxItems: 1
> > > +    description:
> > > +      For RISC-V IOMMU as a platform device, this represents the MMIO base
> > > +      address of registers.
> > > +
> > > +      For RISC-V IOMMU as a PCI device, this represents the PCI-PCI bridge
> > > +      details as described in Documentation/devicetree/bindings/pci/pci.txt
> > > +
> > > +  '#iommu-cells':
> > > +    const: 2
> > > +    description: |
> > > +      Each IOMMU specifier represents the base device ID and number of
> > > +      device IDs.
> > > +
> > > +  interrupts:
> > > +    minItems: 1
> > > +    maxItems: 16
> > > +    description:
> > > +      The presence of this property implies that given RISC-V IOMMU uses
> > > +      wired interrupts to notify the RISC-V HARTS (or CPUs).
> > > +
> > > +  msi-parent:
> > > +    description:
> > > +      The presence of this property implies that given RISC-V IOMMU uses
> > > +      MSIx to notify the RISC-V HARTs (or CPUs). This property should be
> > > +      considered only when the interrupts property is absent.
> > > +
> > > +  dma-coherent:
> > > +    description:
> > > +      Present if page table walks and DMA accessed made by the RISC-V IOMMU
> > > +      are cache coherent with the CPU.
> > > +
> > > +  power-domains:
> > > +    maxItems: 1
> > > +
> >
> > 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.

> Regards,
> Anup
>
> >
> > > +required:
> > > +  - compatible
> > > +  - reg
> > > +  - '#iommu-cells'
> > > +
> > > +additionalProperties: false
> > > +
> > > +examples:
> > > +  - |
> > > +    /* Example 1 (IOMMU platform device with wired interrupts) */
> > > +    immu1: iommu@1bccd000 {
> > > +        compatible = "vendor,chip-iommu", "riscv,iommu";
> > > +        reg = <0x1bccd000 0x1000>;
> > > +        interrupt-parent = <&aplic_smode>;
> > > +        interrupts = <32 4>, <33 4>, <34 4>, <35 4>;
> > > +        #iommu-cells = <2>;
> > > +    };
> > > +
> > > +    /* Device with two IOMMU device IDs, 0 and 7 */
> > > +    master1 {
> > > +        iommus = <&immu1 0 1>, <&immu1 7 1>;
> > > +    };
> > > +
> > > +  - |
> > > +    /* Example 2 (IOMMU platform device with MSIs) */
> > > +    immu2: iommu@1bcdd000 {
> > > +        compatible = "vendor,chip-iommu", "riscv,iommu";
> > > +        reg = <0x1bccd000 0x1000>;
> > > +        msi-parent = <&imsics_smode>;
> > > +        #iommu-cells = <2>;
> > > +    };
> > > +
> > > +    bus {
> > > +        #address-cells = <2>;
> > > +        #size-cells = <2>;
> > > +
> > > +        /* Device with IOMMU device IDs ranging from 32 to 64 */
> > > +        master1 {
> > > +                iommus = <&immu2 32 32>;
> > > +        };
> > > +
> > > +        pcie@40000000 {
> > > +            compatible = "pci-host-cam-generic";
> > > +            device_type = "pci";
> > > +            #address-cells = <3>;
> > > +            #size-cells = <2>;
> > > +            bus-range = <0x0 0x1>;
> > > +
> > > +            /* CPU_PHYSICAL(2)  SIZE(2) */
> > > +            reg = <0x0 0x40000000 0x0 0x1000000>;
> > > +
> > > +            /* BUS_ADDRESS(3)  CPU_PHYSICAL(2)  SIZE(2) */
> > > +            ranges = <0x01000000 0x0 0x01000000  0x0 0x01000000  0x0 0x00010000>,
> > > +                     <0x02000000 0x0 0x41000000  0x0 0x41000000  0x0 0x3f000000>;
> > > +
> > > +            #interrupt-cells = <0x1>;
> > > +
> > > +            /* PCI_DEVICE(3)  INT#(1)  CONTROLLER(PHANDLE)  CONTROLLER_DATA(2) */
> > > +            interrupt-map = <   0x0 0x0 0x0  0x1  &aplic_smode  0x4 0x1>,
> > > +                            < 0x800 0x0 0x0  0x1  &aplic_smode  0x5 0x1>,
> > > +                            <0x1000 0x0 0x0  0x1  &aplic_smode  0x6 0x1>,
> > > +                            <0x1800 0x0 0x0  0x1  &aplic_smode  0x7 0x1>;
> > > +
> > > +            /* PCI_DEVICE(3)  INT#(1) */
> > > +            interrupt-map-mask = <0xf800 0x0 0x0  0x7>;
> > > +
> > > +            msi-parent = <&imsics_smode>;
> > > +
> > > +            /* Devices with bus number 0-127 are mastered via immu2 */
> > > +            iommu-map = <0x0000 &immu2 0x0000 0x8000>;
> > > +        };
> > > +    };
> > > +...
> > > --
> > > 2.34.1
> > >
> > >
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