linux-kernel - Re: local bus enumeration beyond a PCI device

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Date: Thu, 18 Apr 2024 13:45:31 -0500
From: Bjorn Helgaas <helgaas@...nel.org>
To: Chris Packham <Chris.Packham@...iedtelesis.co.nz>
Cc: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
	"linux-acpi@...r.kernel.org" <linux-acpi@...r.kernel.org>,
	Herve Codina <herve.codina@...tlin.com>
Subject: Re: local bus enumeration beyond a PCI device

[+cc Herve]

On Thu, Apr 18, 2024 at 12:24:06AM +0000, Chris Packham wrote:
> Hi,
> 
> We've got a custom x86_64 based design that is using an ASIX9100 to 
> provide a PCI to local bus bridge. Attached to that local bus is an FPGA 
> which mostly provides some GPIOs accessed via registers on the local 
> bus. Right now we've got a custom driver that bundles everything 
> together so effectively we've got a PCI device that provides GPIOs.

What's the local bus?  The ASIX9100 (for which Google doesn't find any
details) would have a PCI interface on the primary (upstream) side.
What's the local bus on the secondary (downstream) side?  Below you
mention "PCI bridge", which normally means both the primary and
secondary sides are PCI buses.

If the local bus is not PCI, I guess the ASIX9100 would look to the OS
like an endpoint, i.e., PCI_HEADER_TYPE_NORMAL, and the ASIX9100
driver would handle any "bridge" functionality completely internally?

Maybe Herve's work at
https://lore.kernel.org/r/20240325153919.199337-1-herve.codina@bootlin.com
would be relevant?

> But as things can change based on the FPGA program I'd like some 
> flexibility to treat it separately from the PCI bridge. So really I'd 
> like to have a PCI device driver for the ASIX9100 that provides a local 
> bus controller and a (platform?) driver for the FPGA that provides the 
> GPIOs where I can have different compatibles for the different 
> implementations.
> 
> Then in the ACPI overlay I'd have something like
> 
>      Scope (\_SB.PCI0.D0B0)
>      {
>          Device (ASIX)
>          {
>              Name (_ADR, 0x0000)
> 
>              Device (FPGA)
>              {
>                          Name (_HID, "PRP0001")
>                          Name (_DSD, Package ()
>                          {
> ToUUID("daffd814-6eba-4d8c-8a91-bc9bbf4aa301"),
>                                      Package ()
>                                      {
>                                                  Package () { 
> "compatible", "my-platform-driver-for-fpga" },
>                                      }
>                          })
>              }
>          }
>      }
> 
>     Scope(\_SB)
>     {
>          Device(OTHR)
>          {
>              GpioIo (Exclusive, PullUp, 0, 0, IoRestrictionInputOnly, 
> "\\_SB.PCI0.D0B0.ASIX.FPGA",) { 0 }
>          }
>     }
> 
> Is it even possible to register a host controller for another platform bus?
> 
> Thanks,
> Chris