linux-kernel - RE: [PATCH 05/15] cxl/acpi: Add probe function to detect restricted CXL hosts in RCD mode

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Message-ID: <6319877d8dc7e_58016294df@dwillia2-xfh.jf.intel.com.notmuch>
Date:   Wed, 7 Sep 2022 23:11:09 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Robert Richter <rrichter@....com>,
        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>
CC:     <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>, Robert Richter <rrichter@....com>
Subject: RE: [PATCH 05/15] cxl/acpi: Add probe function to detect restricted
 CXL hosts in RCD mode

Robert Richter wrote:
> Restricted CXL device (RCD) mode (formerly CXL 1.1) uses a different
> enumeration scheme other than CXL VH (formerly CXL 2.0). In RCD mode a
> host/device (RCH-RCD) pair shows up as a legal PCIe hierarchy with an
> ACPI host bridge ("PNP0A08" or "ACPI0016" HID) and RCiEP connected to
> it with a description of the CXL device.
> 
> Add function cxl_restricted_host_probe() to probe RCD enumerated
> devices. The function implements a loop that detects all CXL capable
> ACPI PCI root bridges in the system (RCD mode only). The iterator
> function cxl_find_next_rch() is introduced to walk through all of the
> CXL hosts. The loop will then enable all CXL devices connected to the
> host. For now, only implement an empty loop with an iterator that
> returns all pci host bridges in the system.
> 
> The probe function is triggered by adding an own root device for RCHs.
> This is different to CXL VH where an ACPI "ACPI0017" root device
> exists. Its detection starts the CXL host detection. In RCD mode such
> a device does not necessarily exists, so solve this by creating a
> plain platform device that is not an ACPI device and is root only for
> RCHs.

These host bridges should be discovered by add_host_bridge_dport(), no?
Unless the BIOS is failing to emit the expected CEDT along with
ACPI0017.