Re: [PATCH v3 1/2] dt-bindings: iommu: add bindings for sprd iommu

[Chunyan Zhang <zhang.lyra@gmail.com>]

On Thu, 11 Feb 2021 at 03:21, Rob Herring <robh@kernel.org> wrote:
>
> On Fri, Feb 5, 2021 at 1:21 AM Chunyan Zhang <zhang.lyra@gmail.com> wrote:
> >
> > Hi Rob,
> >
> > On Fri, 5 Feb 2021 at 07:25, Rob Herring <robh@kernel.org> wrote:
> > >
> > > On Wed, Feb 03, 2021 at 05:07:26PM +0800, Chunyan Zhang wrote:
> > > > From: Chunyan Zhang <chunyan.zhang@unisoc.com>
> > > >
> > > > This iommu module can be used by Unisoc's multimedia devices, such as
> > > > display, Image codec(jpeg) and a few signal processors, including
> > > > VSP(video), GSP(graphic), ISP(image), and CPP(camera pixel processor), etc.
> > > >
> > > > Signed-off-by: Chunyan Zhang <chunyan.zhang@unisoc.com>
> > > > ---
> > > >  .../devicetree/bindings/iommu/sprd,iommu.yaml | 72 +++++++++++++++++++
> > > >  1 file changed, 72 insertions(+)
> > > >  create mode 100644 Documentation/devicetree/bindings/iommu/sprd,iommu.yaml
> > > >
> > > > diff --git a/Documentation/devicetree/bindings/iommu/sprd,iommu.yaml b/Documentation/devicetree/bindings/iommu/sprd,iommu.yaml
> > > > new file mode 100644
> > > > index 000000000000..4fc99e81fa66
> > > > --- /dev/null
> > > > +++ b/Documentation/devicetree/bindings/iommu/sprd,iommu.yaml
> > > > @@ -0,0 +1,72 @@
> > > > +# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
> > > > +# Copyright 2020 Unisoc Inc.
> > > > +%YAML 1.2
> > > > +---
> > > > +$id: http://devicetree.org/schemas/iommu/sprd,iommu.yaml#
> > > > +$schema: http://devicetree.org/meta-schemas/core.yaml#
> > > > +
> > > > +title: Unisoc IOMMU and Multi-media MMU
> > > > +
> > > > +maintainers:
> > > > +  - Chunyan Zhang <zhang.lyra@gmail.com>
> > > > +
> > > > +properties:
> > > > +  compatible:
> > > > +    enum:
> > > > +      - sprd,iommu-v1
> > > > +
> > > > +  "#iommu-cells":
> > > > +    const: 0
> > > > +    description:
> > > > +      Unisoc IOMMUs are all single-master IOMMU devices, therefore no
> > > > +      additional information needs to associate with its master device.
> > > > +      Please refer to the generic bindings document for more details,
> > > > +      Documentation/devicetree/bindings/iommu/iommu.txt
> > > > +
> > > > +  reg:
> > > > +    maxItems: 1
> > > > +    description:
> > > > +      Not required if 'sprd,iommu-regs' is defined.
> > > > +
> > > > +  clocks:
> > > > +    description:
> > > > +      Reference to a gate clock phandle, since access to some of IOMMUs are
> > > > +      controlled by gate clock, but this is not required.
> > > > +
> > > > +  sprd,iommu-regs:
> > > > +    $ref: /schemas/types.yaml#/definitions/phandle-array
> > > > +    description:
> > > > +      Reference to a syscon phandle plus 1 cell, the syscon defines the
> > > > +      register range used by the iommu and the media device, the cell
> > > > +      defines the offset for iommu registers. Since iommu module shares
> > > > +      the same register range with the media device which uses it.
> > > > +
> > > > +required:
> > > > +  - compatible
> > > > +  - "#iommu-cells"
> > > > +
> > > > +oneOf:
> > > > +  - required:
> > > > +      - reg
> > > > +  - required:
> > > > +      - sprd,iommu-regs
> > > > +
> > > > +additionalProperties: false
> > > > +
> > > > +examples:
> > > > +  - |
> > > > +    iommu_disp: iommu-disp {
> > > > +      compatible = "sprd,iommu-v1";
> > > > +      sprd,iommu-regs = <&dpu_regs 0x800>;
> > >
> > > If the IOMMU is contained within another device, then it should just be
> > > a child node of that device.
> >
> > Yes, actually IOMMU can be seen as a child of multimedia devices, I
> > considered moving IOMMU under into multimedia device node, but
> > multimedia devices need IOMMU when probe[1], so I dropped that idea.
>
> Don't design your binding around working-around linux issues.
>
> > And they share the same register base, e.g.
> >
> > +               mm {
> > +                       compatible = "simple-bus";
> > +                       #address-cells = <2>;
> > +                       #size-cells = <2>;
> > +                       ranges;
> > +
> > +                       dpu_regs: syscon@63000000 {
>
> Drop this node.
>
> > +                               compatible = "sprd,sc9863a-dpuregs", "syscon";
> > +                               reg = <0 0x63000000 0 0x1000>;
> > +                       };
> > +
> > +                       dpu: dpu@63000000 {
> > +                               compatible = "sprd,sharkl3-dpu";
> > +                               sprd,disp-regs = <&dpu_regs>;
>
> reg = <0 0x63000000 0 0x800>;
>
> > +                               iommus = <&iommu_dispc>;
> > +                       };
> > +
> > +                       iommu_dispc: iommu@63000000 {
> > +                               compatible = "sprd,iommu-v1";
> > +                               sprd,iommu-regs = <&dpu_regs 0x800>;
>
> reg = <0 0x63000800 0 0x800>;

Alright, considering you deprecate using syscon to map registers here,
I will drop that.
But that would cause the same physical address to be mapped two times at least.
And this is not a single case, since there are a few media devices and
their IOMMUs which all have this issue.

Thanks,
Chunyan

>
> > +                               #iommu-cells = <0>;
>
> Though given it seems there is only 1 client and this might really be
> just 1 h/w block, you don't really need to use the iommu binding at
> all. The DPU should be able to instantiate it's own IOMMU device.
> There's other examples of this such as mali GPU though that is all one
> driver, but that's a Linux implementation detail.
>
> Rob