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

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Date:   Tue, 6 Sep 2022 12:20:35 +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 01.09.22 11:37:57, Jonathan Cameron wrote:
> On Thu, 1 Sep 2022 08:38:52 +0200
> Robert Richter <rrichter@....com> wrote:
> 
> > 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.
> 
> I disagree. CXL spec does not exclude the possibility of real CXL
> RCiEPs. So a CXL 2.0+ device that talks CXL configuration for some
> reason but is part of the root complex itself (maybe a chiplet or
> something where there isn't necessarily a real CXL bus involved).
> Same reason we have RCiEPs in normal PCIe.
> 
> Chasing references - there is only one I can find (CXL r3.0 9.12.1)
> "If a Host bridge is not associated with RCDs or CXL RCiEPs."
> 
> Both listed because they are different things.
> (I think it's fine to say here that this has been queried in
> appropriate place in the past and is something that is allowed).
> 
> So I still don't think the above check is sufficient'. If you
> happen to have just one CXL 2.0+ RCiEP on a host bridge with
> not root ports, then the check will identify it as a restriced
> host.  Maybe I'm missing another check that wouldn't though....
> 
> > There
> > cannot be both types connected to the same host.
> > 
> > Again, see figure 9-12 and 9-13.
> Examples - don't show all the crazy things people are allowed to 
> build - you would need an awful lot of diagrams to do that.

Right, there are references to CXL 2.0+ devices implemented as RCiEPs.

"9.12 CXL VH Enumeration" states that for the CXL Host Bridge
identification the CEDT should be used:

"""
CXL Early Discovery Table (CEDT) may be used to differentiate
between the three software concepts listed above.
"""

This check is added in patch #10 where the RCRB is extracted, so we
are good here.

-Robert