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Date: Thu, 11 Apr 2024 07:39:17 -0500
From: Bjorn Helgaas <helgaas@kernel.org>
To: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Cc: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>,
	Bjorn Helgaas <bhelgaas@google.com>,
	Lorenzo Pieralisi <lpieralisi@kernel.org>,
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Subject: Re: [PATCH v2 2/4] dt-bindings: PCI: mediatek,mt7621: add missing
 child node reg
Message-ID: <20240411123917.GA2180141@bhelgaas>
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On Thu, Apr 11, 2024 at 08:13:18AM +0200, Sergio Paracuellos wrote:
> On Thu, Apr 11, 2024 at 8:01 AM Krzysztof Kozlowski
> <krzysztof.kozlowski@linaro.org> wrote:
> > On 10/04/2024 23:26, Bjorn Helgaas wrote:
> > > On Wed, Apr 10, 2024 at 08:15:19PM +0200, Krzysztof Kozlowski wrote:
> > >> MT7621 PCI host bridge has children which apparently are also PCI host
> > >> bridges, at least that's what the binding suggest.
> > >
> > > What does it even mean for a PCI host bridge to have a child that is
> > > also a PCI host bridge?
> > >
> > > Does this mean a driver binds to the "parent" host bridge, enumerates
> > > the PCI devices below it, and finds a "child" host bridge?
> 
> Yes, that is exactly what you can see on enumeration.
> 
> The following is a typical boot trace where all bridges has a device also below:
> 
> mt7621-pci 1e140000.pcie: host bridge /pcie@1e140000 ranges:
> mt7621-pci 1e140000.pcie:   No bus range found for /pcie@1e140000, using [bus 00-ff]
> mt7621-pci 1e140000.pcie:      MEM 0x0060000000..0x006fffffff -> 0x0060000000
> mt7621-pci 1e140000.pcie:       IO 0x001e160000..0x001e16ffff -> 0x0000000000
> mt7621-pci 1e140000.pcie: PCIE0 enabled
> mt7621-pci 1e140000.pcie: PCIE1 enabled
> mt7621-pci 1e140000.pcie: PCIE2 enabled
> mt7621-pci 1e140000.pcie: PCI host bridge to bus 0000:00

1e140000.pcie is a host bridge.  It has some CPU-specific bus on the
upstream side, standard PCI (domain 0000, buses 00-ff) on the
downstream side.

> pci 0000:00:00.0: [0e8d:0801] type 01 class 0x060400
> pci 0000:00:01.0: [0e8d:0801] type 01 class 0x060400
> pci 0000:00:02.0: [0e8d:0801] type 01 class 0x060400

> pci 0000:01:00.0: [1b21:0611] type 00 class 0x010185

> pci 0000:00:00.0: PCI bridge to [bus 01-ff]
> pci 0000:00:00.0:   bridge window [io  0x0000-0x0fff]
> pci 0000:00:00.0:   bridge window [mem 0x00000000-0x000fffff]
> pci 0000:00:00.0:   bridge window [mem 0x00000000-0x000fffff pref]

00:00.0 looks like a PCIe Root Port to bus 01.  This is not a host
bridge; it's just a standard PCI-to-PCI bridge with PCI on both the
upstream and downstream sides.

> pci 0000:02:00.0: [1b21:0611] type 00 class 0x010185

> pci 0000:00:01.0: PCI bridge to [bus 02-ff]
> pci 0000:00:01.0:   bridge window [io  0x0000-0x0fff]
> pci 0000:00:01.0:   bridge window [mem 0x00000000-0x000fffff]
> pci 0000:00:01.0:   bridge window [mem 0x00000000-0x000fffff pref]

00:01.0 is another Root Port to bus 02.

> pci 0000:03:00.0: [1b21:0611] type 00 class 0x010185

> pci 0000:00:02.0: PCI bridge to [bus 03-ff]
> pci 0000:00:02.0:   bridge window [io  0x0000-0x0fff]
> pci 0000:00:02.0:   bridge window [mem 0x00000000-0x000fffff]
> pci 0000:00:02.0:   bridge window [mem 0x00000000-0x000fffff pref]
> pci_bus 0000:03: busn_res: [bus 03-ff] end is updated to 03

And 00:02.0 is a third Root Port to bus 03.

> pci 0000:00:00.0: PCI bridge to [bus 01]
> pci 0000:00:00.0:   bridge window [io  0x0000-0x0fff]
> pci 0000:00:00.0:   bridge window [mem 0x60000000-0x600fffff]
> pci 0000:00:00.0:   bridge window [mem 0x60100000-0x601fffff pref]
> pci 0000:00:01.0: PCI bridge to [bus 02]
> pci 0000:00:01.0:   bridge window [io  0x1000-0x1fff]
> pci 0000:00:01.0:   bridge window [mem 0x60200000-0x602fffff]
> pci 0000:00:01.0:   bridge window [mem 0x60300000-0x603fffff pref]
> pci 0000:00:02.0: PCI bridge to [bus 03]
> pci 0000:00:02.0:   bridge window [io  0x2000-0x2fff]
> pci 0000:00:02.0:   bridge window [mem 0x60400000-0x604fffff]
> 
> > I think the question should be towards Mediatek folks. I don't know what
> > this hardware is exactly, just looks like pci-pci-bridge. The driver
> > calls the children host bridges as "ports".
> 
> You can see the topology here in my first driver submit cover letter
> message [0].
> 
>  [0]: https://lore.kernel.org/all/CAMhs-H-BA+KzEwuDPzcmrDPdgJBFA2XdYTBvT4R4MEOUB=WQ1g@mail.gmail.com/t/

Nothing unusual here, this looks like the standard PCIe topology.

What *might* be unusual is describing the Root Ports in DT.  Since
they are standard PCI devices, they shouldn't need DT description
unless there's some unusual power/clock/reset control or something
that is not discoverable via PCI enumeration.

Bjorn