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: <631984f6d7c92_58016294e6@dwillia2-xfh.jf.intel.com.notmuch>
Date:   Wed, 7 Sep 2022 23:00:22 -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.

As I mentioned in the cover letter a BIOS that does not provide an
ACPI0017 device is opting out of the possibility OS first error handling
and other OS CXL services. ACPI0017 is mandatory.

Otherwise, its odd to have a module create the device that its driver is
going to drive. That's a backwards driver model and why we proposed
ACPI0017 in the first instance.

> 
> Signed-off-by: Robert Richter <rrichter@....com>
> ---
>  drivers/cxl/acpi.c | 71 +++++++++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 70 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/cxl/acpi.c b/drivers/cxl/acpi.c
> index 31e104f0210f..a19e3154dd44 100644
> --- a/drivers/cxl/acpi.c
> +++ b/drivers/cxl/acpi.c
> @@ -312,6 +312,33 @@ static int add_root_nvdimm_bridge(struct device *match, void *data)
>  	return 1;
>  }
>  
> +struct pci_host_bridge *cxl_find_next_rch(struct pci_host_bridge *host)
> +{
> +	struct pci_bus *bus = host ? host->bus : NULL;
> +
> +	while ((bus = pci_find_next_bus(bus)) != NULL) {
> +		host = bus ? to_pci_host_bridge(bus->bridge) : NULL;
> +		if (!host)
> +			continue;
> +
> +		dev_dbg(&host->dev, "PCI bridge found\n");
> +
> +		return host;
> +	}
> +
> +	return NULL;
> +}
> +
> +static int __init cxl_restricted_host_probe(struct platform_device *pdev)
> +{
> +	struct pci_host_bridge *host = NULL;
> +
> +	while ((host = cxl_find_next_rch(host)) != NULL) {
> +	}
> +
> +	return 0;
> +}
> +
>  static struct lock_class_key cxl_root_key;
>  
>  static void cxl_acpi_lock_reset_class(void *dev)
> @@ -445,6 +472,13 @@ static int cxl_acpi_probe(struct platform_device *pdev)
>  	struct acpi_device *adev = ACPI_COMPANION(host);
>  	struct cxl_cfmws_context ctx;
>  
> +	/*
> +	 * For RCH (CXL 1.1 hosts) the probe is triggered by a plain
> +	 * platform dev which does not have an acpi companion.
> +	 */
> +	if (!adev)
> +		return cxl_restricted_host_probe(pdev);
> +
>  	device_lock_set_class(&pdev->dev, &cxl_root_key);
>  	rc = devm_add_action_or_reset(&pdev->dev, cxl_acpi_lock_reset_class,
>  				      &pdev->dev);
> @@ -518,6 +552,7 @@ MODULE_DEVICE_TABLE(acpi, cxl_acpi_ids);
>  
>  static const struct platform_device_id cxl_test_ids[] = {
>  	{ "cxl_acpi" },
> +	{ "cxl_root" },
>  	{ },
>  };
>  MODULE_DEVICE_TABLE(platform, cxl_test_ids);
> @@ -531,7 +566,41 @@ static struct platform_driver cxl_acpi_driver = {
>  	.id_table = cxl_test_ids,
>  };
>  
> -module_platform_driver(cxl_acpi_driver);
> +static void cxl_acpi_device_release(struct device *dev) { }
> +
> +static struct platform_device cxl_acpi_device = {
> +	.name = "cxl_root",
> +	.id = PLATFORM_DEVID_NONE,
> +	.dev = {
> +		.release = cxl_acpi_device_release,
> +	}
> +};
> +
> +static int __init cxl_host_init(void)
> +{
> +	int rc;
> +
> +	/* Kick off restricted host (CXL 1.1) detection */
> +	rc = platform_device_register(&cxl_acpi_device);
> +	if (rc) {
> +		platform_device_put(&cxl_acpi_device);
> +		return rc;
> +	}
> +	rc = platform_driver_register(&cxl_acpi_driver);
> +	if (rc)
> +		platform_device_unregister(&cxl_acpi_device);
> +	return rc;
> +}
> +
> +static void __exit cxl_host_exit(void)
> +{
> +	platform_driver_unregister(&cxl_acpi_driver);
> +	platform_device_unregister(&cxl_acpi_device);
> +}
> +
> +module_init(cxl_host_init);
> +module_exit(cxl_host_exit);
> +
>  MODULE_LICENSE("GPL v2");
>  MODULE_IMPORT_NS(CXL);
>  MODULE_IMPORT_NS(ACPI);
> -- 
> 2.30.2
>