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

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Date:   Thu, 1 Sep 2022 11:23:48 +0100
From:   Jonathan Cameron <Jonathan.Cameron@...wei.com>
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>
Subject: Re: [PATCH 08/15] cxl/acpi: Check RCH's CXL DVSEC capabilities

On Thu, 1 Sep 2022 08:30:49 +0200
Robert Richter <rrichter@....com> wrote:

> On 31.08.22 11:52:24, Jonathan Cameron wrote:
> > On Wed, 31 Aug 2022 10:15:56 +0200
> > Robert Richter <rrichter@....com> wrote:  
> 
> > > diff --git a/drivers/cxl/acpi.c b/drivers/cxl/acpi.c
> > > index ffdf439adb87..f9cdf23a91a8 100644
> > > --- a/drivers/cxl/acpi.c
> > > +++ b/drivers/cxl/acpi.c
> > > @@ -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.  
> 
> The software view of an RCD is always the same, it shows up always as
> an RCiEP. See figure 9-12 and 9-13 of the spec.

Ah. I see I misread the following from CXL r3.0 9.11.4 

"This ACPI Host Bridge spawns a legal PCIe hierarchy. All PCIe Endpoints located in the 
RCD are children of this ACPI Host Bridge. These Endpoints may appear directly on the 
Root bus number or may appear behind a Root Port located on the Root bus."

I guess that is allowing 'additional' PCIe endpoints below a root port attached
to the ACPI Host Bridge, but not dev 0 func 0.  That's a subtle distinction
I missed.


> 
> -Robert
> 
> >   
> > > +		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);
> > > +		pci_dev_put(pdev);
> > > +
> > > +		if (!is_restricted_host)
> > > +			continue;
> > > +
> > > +		dev_dbg(&host->dev, "CXL restricted host found\n");
> > > +
> > >  		return host;
> > >  	}