linux-kernel - Re: [PATCH v3 1/9] cxl/acpi: Register CXL host ports by bridge device

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Message-ID: <20221109231124.GA581464@bhelgaas>
Date:   Wed, 9 Nov 2022 17:11:24 -0600
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Robert Richter <rrichter@....com>
Cc:     Alison Schofield <alison.schofield@...el.com>,
        Vishal Verma <vishal.l.verma@...el.com>,
        Ira Weiny <ira.weiny@...el.com>,
        Ben Widawsky <bwidawsk@...nel.org>,
        Dan Williams <dan.j.williams@...el.com>,
        linux-cxl@...r.kernel.org, linux-kernel@...r.kernel.org,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Len Brown <lenb@...nel.org>,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>,
        Davidlohr Bueso <dave@...olabs.net>,
        Dave Jiang <dave.jiang@...el.com>
Subject: Re: [PATCH v3 1/9] cxl/acpi: Register CXL host ports by bridge device

On Wed, Nov 09, 2022 at 11:40:51AM +0100, Robert Richter wrote:
> A port of a CXL host bridge links to the bridge's acpi device

s/acpi/ACPI/ to match usage below.

> (&adev->dev) with its corresponding uport/dport device (uport_dev and
> dport_dev respectively). The device is not a direct parent device in
> the PCI topology as pdev->dev.parent points to a PCI bridge's (struct
> pci_host_bridge) device. The following CXL memory device hierarchy
> would be valid for an endpoint once an RCD EP would be enabled (note
> this will be done in a later patch):
> 
> VH mode:
> 
>  cxlmd->dev.parent->parent
>         ^^^\^^^^^^\ ^^^^^^\
>             \      \       pci_dev (Type 1, Downstream Port)
>              \      pci_dev (Type 0, PCI Express Endpoint)
>               cxl mem device
> 
> RCD mode:
> 
>  cxlmd->dev.parent->parent
>         ^^^\^^^^^^\ ^^^^^^\
>             \      \       pci_host_bridge
>              \      pci_dev (Type 0, RCiEP)
>               cxl mem device
> 
> In VH mode a downstream port is created by port enumeration and thus
> always exists.
> 
> Now, in RCD mode the host bridge also already exists but it references
> to an ACPI device. A port lookup by the PCI device's parent device
> will fail as a direct link to the registered port is missing. The ACPI
> device of the bridge must be determined first.
> 
> To prevent this, change port registration of a CXL host to use the
> bridge device instead. Do this also for the VH case as port topology
> will better reflect the PCI topology then.
> 
> If a mock device is registered by a test driver, the bridge pointer
> can be NULL. Keep using the matching ACPI device (&adev->dev) as a
> fallback in this case.
> 
> Signed-off-by: Robert Richter <rrichter@....com>