Re: [PATCH v2 1/5] dt-bindings: PCI: Add STM32MP25 PCIe root complex bindings

[Bjorn Helgaas <helgaas@kernel.org>]

[cc->to: Rob for RC/RP separation conversation]

On Thu, Dec 05, 2024 at 02:41:26PM +0100, Christian Bruel wrote:
> On 12/3/24 23:25, Bjorn Helgaas wrote:
> > On Tue, Nov 26, 2024 at 04:51:15PM +0100, Christian Bruel wrote:
> > > Document the bindings for STM32MP25 PCIe Controller configured in
> > > root complex mode.
> > > 
> > > Supports 4 legacy interrupts and MSI interrupts from the ARM
> > > GICv2m controller.

> > > +  wake-gpios:
> > > +    description: GPIO controlled connection to WAKE# input signal
> > 
> > I'm not a hardware guy, but this sounds like a GPIO that *reads*
> > WAKE#, not controls it.
> 
> Rephrasing as
> "GPIO used as WAKE# input signal" (output for the endpoint bindings)

Perfect, that makes a lot of sense.

> > > +    pcie@48400000 {
> > > +        compatible = "st,stm32mp25-pcie-rc";
> > > +        device_type = "pci";
> > > +        num-lanes = <1>;
> > 
> > num-lanes applies to a Root Port, not to a Root Complex.  I know most
> > bindings conflate Root Ports with the Root Complex, maybe because many
> > of these controllers only support a single Root Port?
> > 
> > But are we ever going to separate these out?  I assume someday
> > controllers will support multiple Root Ports and/or additional devices
> > on the root bus, like RCiEPs, RCECs, etc., and we'll need per-RP phys,
> > max-link-speed, num-lanes, reset-gpios, etc.
> > 
> > Seems like it would be to our benefit to split out the Root Ports when
> > we can, even if the current hardware only supports one, so we can
> > start untangling the code and data structures.
> 
> OK. and we support only 1 lane anyway, so drop it.

Makes sense.  What about phys, resets, etc?  I'm pretty sure a PHY
would be a per-Root Port thing, and some resets and wakeup signals
also.

For new drivers, I think we should start adding Root Port stanzas to
specifically associate those things with the Root Port, e.g.,
something like this?

  pcie@48400000 {
    compatible = "st,stm32mp25-pcie-rc";

    pcie@0,0 {
      reg = <0x0000 0 0 0 0>;
      phys = <&combophy PHY_TYPE_PCIE>;
      phy-names = "pcie-phy";
    };
  };

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/devicetree/bindings/pci/mediatek,mt7621-pcie.yaml?id=v6.12#n111
is one binding that does this, others include apple,pcie.yaml,
brcm,stb-pcie.yaml, hisilicon,kirin-pcie.yaml.

Bjorn