linux-kernel - Re: [PATCH 08/15] cxl/acpi: Check RCH's CXL DVSEC capabilities

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Message-ID: <YxBTfD+4hDgo6Sah@rric.localdomain>
Date:   Thu, 1 Sep 2022 08:38:52 +0200
From:   Robert Richter <rrichter@....com>
To:     Jonathan Cameron <Jonathan.Cameron@...wei.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>
Subject: Re: [PATCH 08/15] cxl/acpi: Check RCH's CXL DVSEC capabilities

On 31.08.22 12:12:22, Jonathan Cameron wrote:
> > On Wed, 31 Aug 2022 10:15:56 +0200
> > Robert Richter <rrichter@....com> wrote:

> > > @@ -322,6 +322,8 @@ struct pci_host_bridge *cxl_find_next_rch(struct pci_host_bridge *host)
> > >  {
> > >  	struct pci_bus *bus = host ? host->bus : NULL;
> > >  	struct acpi_device *adev;
> > > +	struct pci_dev *pdev;
> > > +	bool is_restricted_host;
> > >  
> > >  	while ((bus = pci_find_next_bus(bus)) != NULL) {
> > >  		host = bus ? to_pci_host_bridge(bus->bridge) : NULL;
> > > @@ -343,6 +345,20 @@ struct pci_host_bridge *cxl_find_next_rch(struct pci_host_bridge *host)
> > >  		dev_dbg(&host->dev, "PCI ACPI host found: %s\n",
> > >  			acpi_dev_name(adev));
> > >  
> > > +		/* Check CXL DVSEC of dev 0 func 0 */  
> > 
> > So assumption here is that the hostbridge has a one or more RCiEPs.
> > The spec (r3.0 9.11.4) allows for the EP to appear behind a root port
> > - that case always felt odd to me, so I'm fine with not supporting it until
> > we see a user.
> > 
> > > +		pdev = pci_get_slot(bus, PCI_DEVFN(0, 0));
> > > +		is_restricted_host = pdev
> > > +			&& (pci_pcie_type(pdev) == PCI_EXP_TYPE_RC_END)
> > > +			&& pci_find_dvsec_capability(pdev,
> > > +						PCI_DVSEC_VENDOR_ID_CXL,
> > > +						CXL_DVSEC_PCIE_DEVICE);
> 
> Thinking a bit more on this.  I'm not sure this is sufficient.
> Nothing in CXL 2.0 or later prevents true RCiEP devices (there are a
> few references in CXL 3.0 e.g. 9.12.1 has RCDs or CXL RCiEPs - so just
> detecting that there is one on the host bridge might not be sufficient
> to distinguish this from a non RCH / RCB.

An RCD has its own host bridge created (software view, not the phys
topology). Host and device are paired in this case. Non-RCDs are
standard endpoints and not RCiEPs, they have their own host. There
cannot be both types connected to the same host.

Again, see figure 9-12 and 9-13.

-Robert

> 
> > > +		pci_dev_put(pdev);
> > > +
> > > +		if (!is_restricted_host)
> > > +			continue;
> > > +
> > > +		dev_dbg(&host->dev, "CXL restricted host found\n");
> > > +
> > >  		return host;
> > >  	}
> > >  
> > > @@ -354,6 +370,7 @@ 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) {
> > > +		dev_info(&host->dev, "host supports CXL\n");
> > >  	}
> > >  
> > >  	return 0;  
> > 
>