Date:   Sun, 01 Aug 2021 10:31:18 +0100
From:   Marc Zyngier <maz@...nel.org>
To:     Rob Herring <robh@...nel.org>
Cc:     Mark Kettenis <mark.kettenis@...all.nl>,
        devicetree@...r.kernel.org, robin.murphy@....com,
        sven@...npeter.dev, Mark Kettenis <kettenis@...nbsd.org>,
        Hector Martin <marcan@...can.st>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        linux-arm-kernel@...ts.infradead.org, linux-pci@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 1/2] dt-bindings: pci: Add DT bindings for apple,pcie

On Tue, 27 Jul 2021 00:18:48 +0100,
Rob Herring <robh@...nel.org> wrote:
> 
> On Mon, Jul 26, 2021 at 10:32:00AM +0200, Mark Kettenis wrote:
> > From: Mark Kettenis <kettenis@...nbsd.org>
> > 
> > The Apple PCIe host controller is a PCIe host controller with
> > multiple root ports present in Apple ARM SoC platforms, including
> > various iPhone and iPad devices and the "Apple Silicon" Macs.
> > 
> > Signed-off-by: Mark Kettenis <kettenis@...nbsd.org>
> > ---
> >  .../devicetree/bindings/pci/apple,pcie.yaml   | 166 ++++++++++++++++++
> >  MAINTAINERS                                   |   1 +
> >  2 files changed, 167 insertions(+)
> >  create mode 100644 Documentation/devicetree/bindings/pci/apple,pcie.yaml
> > 
> > diff --git a/Documentation/devicetree/bindings/pci/apple,pcie.yaml b/Documentation/devicetree/bindings/pci/apple,pcie.yaml
> > new file mode 100644
> > index 000000000000..bfcbdee79c64
> > --- /dev/null
> > +++ b/Documentation/devicetree/bindings/pci/apple,pcie.yaml
> > @@ -0,0 +1,166 @@
> > +# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
> > +%YAML 1.2
> > +---
> > +$id: http://devicetree.org/schemas/pci/apple,pcie.yaml#
> > +$schema: http://devicetree.org/meta-schemas/core.yaml#
> > +
> > +title: Apple PCIe host controller
> > +
> > +maintainers:
> > +  - Mark Kettenis <kettenis@...nbsd.org>
> > +
> > +description: |
> > +  The Apple PCIe host controller is a PCIe host controller with
> > +  multiple root ports present in Apple ARM SoC platforms, including
> > +  various iPhone and iPad devices and the "Apple Silicon" Macs.
> > +  The controller incorporates Synopsys DesigWare PCIe logic to
> > +  implements its root ports.  But the ATU found on most DesignWare
> > +  PCIe host bridges is absent.
> 
> blank line
> 
> > +  All root ports share a single ECAM space, but separate GPIOs are
> > +  used to take the PCI devices on those ports out of reset.  Therefore
> > +  the standard "reset-gpio" and "max-link-speed" properties appear on
> 
> reset-gpios
> 
> > +  the child nodes that represent the PCI bridges that correspond to
> > +  the individual root ports.
> 
> blank line
> 
> > +  MSIs are handled by the PCIe controller and translated into regular
> > +  interrupts.  A range of 32 MSIs is provided.  These 32 MSIs can be
> > +  distributed over the root ports as the OS sees fit by programming
> > +  the PCIe controller's port registers.
> > +
> > +allOf:
> > +  - $ref: /schemas/pci/pci-bus.yaml#
> > +
> > +properties:
> > +  compatible:
> > +    items:
> > +      - const: apple,t8103-pcie
> > +      - const: apple,pcie
> > +
> > +  reg:
> > +    minItems: 3
> > +    maxItems: 5
> > +
> > +  reg-names:
> > +    minItems: 3
> > +    maxItems: 5
> > +    items:
> > +      - const: config
> > +      - const: rc
> > +      - const: port0
> > +      - const: port1
> > +      - const: port2
> > +
> > +  ranges:
> > +    minItems: 2
> > +    maxItems: 2
> > +
> > +  interrupts:
> > +    description:
> > +      Interrupt specifiers, one for each root port.
> > +    minItems: 1
> > +    maxItems: 3
> > +
> > +  msi-controller: true
> > +  msi-parent: true
> > +
> > +  msi-ranges:
> > +    description:
> > +      A list of pairs <intid span>, where "intid" is the first
> > +      interrupt number that can be used as an MSI, and "span" the size
> > +      of that range.
> > +    $ref: /schemas/types.yaml#/definitions/uint32-matrix
> > +    items:
> > +      minItems: 2
> > +      maxItems: 2
> 
> I still have issues I raised on v1 with this property. It's genericish 
> looking, but not generic. 'intid' as a single cell can't specify any 
> parent interrupt such as a GIC which uses 3 cells. You could put in all 
> the cells, but you'd still be assuming which cell you can increment.

The GIC bindings already use similar abstractions, see what we do for
both GICv2m and GICv3 MBIs. Other MSI controllers use similar
properties (alpine and loongson, for example).

> I think you should just list all these under 'interrupts' using 
> interrupt-names to make your life easier:
> 
> interrupt-names:
>   items:
>     - const: port0
>     - const: port1
>     - const: port2
>     - const: msi0
>     - const: msi1
>     - const: msi2
>     - const: msi3
>     ...
> 
> Yeah, it's kind of verbose, but if the h/w block handles N interrupts, 
> you should list N interrupts. The worst case for the above is N entries 
> too if not contiguous.

And that's where I beg to differ, again.

Specifying interrupts like this gives the false impression that these
interrupts are generated by the device that owns them (the RC). Which
for MSIs is not the case. This is not only verbose, this is
semantically dubious. And what should we do when the number of
possible interrupt is ridiculously large, as it is for the GICv3 ITS?

I wish we had a standard way to express these constraints. Until we
do, I don't think enumerating individual interrupts is a practical
thing to do, nor that it actually represents the topology of the
system.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.

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