[PATCH v2 1/2] dt-bindings: i3c: MIPI I3C Host Controller Interface

[Rob Herring]

On Tue, Aug 25, 2020 at 4:02 PM Nicolas Pitre <nico at fluxnic.net> wrote:
>
> On Tue, 25 Aug 2020, Rob Herring wrote:
>
> > On Tue, Aug 18, 2020 at 11:17:22PM -0400, Nicolas Pitre wrote:
> > > From: Nicolas Pitre <npitre at baylibre.com>
> > >
> > > The MIPI I3C HCI (Host Controller Interface) specification defines
> > > a common software driver interface to support compliant MIPI I3C
> > > host controller hardware implementations from multiple vendors.
> > >
> > > Signed-off-by: Nicolas Pitre <npitre at baylibre.com>
> > > ---
> > >  .../devicetree/bindings/i3c/mipi-i3c-hci.yaml | 41 +++++++++++++++++++
> > >  1 file changed, 41 insertions(+)
> > >  create mode 100644 Documentation/devicetree/bindings/i3c/mipi-i3c-hci.yaml
> > >
> > > diff --git a/Documentation/devicetree/bindings/i3c/mipi-i3c-hci.yaml b/Documentation/devicetree/bindings/i3c/mipi-i3c-hci.yaml
> > > new file mode 100644
> > > index 0000000000..8fc18ea922
> > > --- /dev/null
> > > +++ b/Documentation/devicetree/bindings/i3c/mipi-i3c-hci.yaml
> > > @@ -0,0 +1,41 @@
> > > +# SPDX-License-Identifier: (GPL-2.0 OR BSD-2-Clause)
> > > +%YAML 1.2
> > > +---
> > > +$id: "http://devicetree.org/schemas/i3c/mipi-i3c-hci.yaml#"
> > > +$schema: "http://devicetree.org/meta-schemas/core.yaml#"
> > > +
> > > +title: MIPI I3C HCI Device Tree Bindings
> > > +
> > > +maintainers:
> > > +  - Nicolas Pitre <npitre at baylibre.com>
> > > +
> > > +description: |
> > > +  MIPI I3C Host Controller Interface
> > > +
> > > +  The MIPI I3C HCI (Host Controller Interface) specification defines
> > > +  a common software driver interface to support compliant MIPI I3C
> > > +  host controller hardware implementations from multiple vendors.
> > > +
> > > +  For details, please see:
> > > +  https://www.mipi.org/specifications/i3c-hci
> > > +
> > > +properties:
> > > +  compatible:
> > > +    const: mipi-i3c-hci
> >
> > What about my comments on v1? Pasted again:
>
> Oops, sorry, I missed them.
>
> > A register interface (or protocol) spec is never complete enough to
> > capture all the details about a specific h/w implementation. One just
> > has to go look at AHCI, EHCI, OHCI, XHCI, UFS, 8250, etc. bindings.
> > Let's not start with pretending that here. Fine for this to be a
> > fallback, but it must have a compatible for a specific implementation.
>
> You might have to indulge me a bit as I don't
> understand what you're asking.
>
> Currently there are very few implementations. One of them lives in an
> FPGA and the example below is actually the DT entry I use for it. I'm
> guessing specific vendor implementations will have their own tweaks
> eventually, such as clock sources and whatnot.

Yes, exactly. And bugs too.

> But that is outside of
> the spec (actually the spec defines a register area for eventual vendor
> specific usage). But I have no visibility into that and of course the
> code has no provision for that yet either.
>
> So I imagine there will be something like this in dts files eventually:
>
>         compatibvle = "intel,foobar_soc_i3c_hci", "mipi-i3c-hci";
>
> Is that what you mean?

Yes. Even your FPGA is tied to some implementation...

> > Also, which version of the spec does this compatible correspond to?
>
> All of them.
>
> > Or are there not HCI differences in the spec versions you mention in
> > the cover letter?
>
> The hardware is self advertising per the spec. So there is no need to
> carry such distinction in the DT compatible. Even vendor extensions are
> tagged with MIPI vendor IDs in the hardware directly.

Oh good, folks are learning. :)

Is the vendor ID (and revision) discoverable even if no vendor
extensions? If so, then I'm more comfortable with "mipi-i3c-hci" on
it's own. The exception will be if there's setup needed to discover
the h/w which seems likely. In that case, we should probably do
compatible strings based on VID/PID like PCI, USB, etc. No need to
define that now I guess, but please add some sort of summary of the
above about the discoverability of the HCI implementer and features.

Rob