Re: [PATCH v3 1/2] dt-bindings: pci: Add DT bindings for apple,pcie

[Rob Herring <robh@kernel.org>]

On Sun, Aug 15, 2021 at 11:36 AM Marc Zyngier <maz@kernel.org> wrote:
>
> Hi Rob,
>
> Apologies for the delay, I somehow misplaced this email...
>
> On Mon, 02 Aug 2021 17:10:39 +0100,
> Rob Herring <robh@kernel.org> wrote:
> >
> > On Sun, Aug 1, 2021 at 3:31 AM Marc Zyngier <maz@kernel.org> wrote:
> > >
> > > On Tue, 27 Jul 2021 00:18:48 +0100,
> > > Rob Herring <robh@kernel.org> wrote:
> > > >
> > > > On Mon, Jul 26, 2021 at 10:32:00AM +0200, Mark Kettenis wrote:
> > > > > From: Mark Kettenis <kettenis@openbsd.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@openbsd.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@openbsd.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).
> >
> > That's the problem. Everyone making up their own crap.
>
> And that crap gets approved:
>
> https://lore.kernel.org/lkml/20200512205704.GA10412@bogus/
>
> I'm not trying to be antagonistic here, but it seems that your
> position on this very subject has changed recently.

Not really, I think it's not the first time we've discussed this. But
as I see things over and over, my tolerance for another instance
without solving the problem for everyone diminishes. And what other
leverage do I have?

Additionally, how long we have to support something comes into play. I
have no idea for a Loongson MSI controller. I have a better idea on an
Apple product...

> > > > 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.
> >
> > It's no different than an interrupt controller node having an
> > interrupts property. The source is downstream and the interrupt
> > controller is combining/translating the interrupts.
> >
> > The physical interrupt signals are connected to and originating in
> > this block.
>
> Oh, I also object to this, for the same reasons. The only case where
> it makes sense IMHO is when the interrupt controller is a multiplexer.

So we've had the same kind of property for interrupt multiplexers. I'm
fine if you think an 'MSI to interrupts mapping property' should be
named something else.

> > That sounds like perfectly 'describing the h/w' to me.
>
> I guess we have a different view of about these things. At the end of
> the day, I don't care enough as long as we can expose a range of
> interrupts one way or another.

I don't really either. I just don't want 10 ways AND another...

> > > 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 don't disagree with the verbose part. But that's not really an issue
> > in this case.
> >
> > > 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.
> >
> > The only way a standard way will happen is to stop accepting the
> > custom properties.
> >
> > All the custom properties suffer from knowledge of what the parent
> > interrupt controller is. To fix that, I think we need something like
> > this:
> >
> > msi-ranges = <intspec base>, <intspec step>, <intspec end>;
> >
> > 'intspec' is defined by the parent interrupt-controller cells. step is
> > the value to add. And end is what to match on to stop aka the last
> > interrupt in the range. For example, if the GIC is the parent, we'd
> > have something like this:
> >
> > <GIC_SPI 123 0>, <0 1 0>, <GIC_SPI 124 0>
> >
> > Does this apply to cases other than MSI? I think so as don't we have
> > the same type of properties with the low power mode shadow interrupt
> > controllers?  So 'interrupt-ranges'?
>
> This would work, though the increment seems a bit over-engineered. You
> also may need this property to accept multiple ranges.

Yes, certainly. Worst case is a map.

> > It looks to me like there's an assumption in the kernel that an MSI
> > controller has a linear range of parent interrupts? Is that correct
> > and something that's guaranteed? That assumption leaks into the
> > existing bindings.
>
> Depends on how the controller works. In general, the range maps to the
> MultiMSI requirements where the message is an offset from the base of
> the interrupt range. So you generally end-up with ranges of at least
> 32 contiguous MSIs. Anything under that is sub-par and probably not
> worth supporting.

Maybe just this is enough:
msi-ranges = <intspec base>, <length>, <intspec base>, <length>, ...

While I say 'length' here, that's really up to the interrupt parent to
interpret the intspec cells.

> Of course, the controller may have some mapping facilities, which
> makes things more... interesting.
>
> > It's fine for the kernel to assume that until there's a case that's
> > not linear, but a common binding needs to be able handle a
> > non-linear case.
>
> Fair enough. I can probably work with Mark to upgrade the binding and
> the M1 PCIe code. Could you come up with a more formalised proposal?

Not my itch.

Rob