Re: [PATCH v2 1/3] PCI: Add check for CXL Secondary Bus Reset

Bjorn Helgaas
2024-03-28T19:14:59Z

On Thu, Mar 28, 2024 at 12:03:17PM -0700, Dan Williams wrote:
> Bjorn Helgaas wrote:
> > On Wed, Mar 27, 2024 at 04:57:40PM -0700, Dave Jiang wrote:
> > > On 3/27/24 2:26 PM, Bjorn Helgaas wrote:
> > > > On Mon, Mar 25, 2024 at 04:58:01PM -0700, Dave Jiang wrote:

> > > >> +/* Compute Express Link (CXL) */
> > > >> +#define PCI_DVSEC_VENDOR_ID_CXL 0x1e98
> > > >
> > > > I see that you're just moving this #define from drivers/cxl/cxlpci.h,
> > > > but I'm scratching my head a bit. As used here, this is to match the
> > > > DVSEC Vendor ID (PCIe r6.0, sec 7.9.6.2).
> > > >
> > > > IIUC, this should be just a PCI SIG-defined "Vendor ID", e.g.,
> > > > "PCI_VENDOR_ID_CXL", that doesn't need the "DVSEC" qualifier in the
> > > > name, and would normally be defined in include/linux/pci_ids.h.
> > > >
> > > > But I don't see CXL in pci_ids.h, and I don't see either "CXL" or the
> > > > value "1e98" in the PCI SIG list at
> > > > https://pcisig.com/membership/member-companies.
> > > >
> > > I'll create a new patch and move to include/linux/pci_ids.h first
> > > for this define and change to PCI_VENDOR_ID_CXL. The value is
> > > defined in CXL spec r3.1 sec 8.1.1.
> >
> > I saw the CXL mentions of 0x1e98, but IMO that's not an authoritative
> > source; no vendor is allowed to just squat on a Vendor ID value simply
> > by mentioning it in their own internal specs. That would obviously
> > lead to madness.
> >
> > The footnote in CXL r3.1, sec 3.1.2, about how the 1E98h value is only
> > for use in DVSEC etc., is really weird.
> >
> > IIUC, the PCI SIG controls the Vendor ID namespace, so I'm still
> > really confused about why it is not reserved there. I emailed the PCI
> > SIG, but the footnote suggests to me that there some kind of history
> > here that I don't know.
> >
> > Anyway, I think all we can do here is to put the definition in
> > include/linux/pci_ids.h as you did and hope 0x1e98 is never allocated
> > to another vendor.
>
> Oh, true, I think this should be PCI_DVSEC_VENDOR_ID_CXL, because afaics
> it is still possible that 0x1e98 be used as a non-DVSEC vendor-id in
> some future device.

What a disaster that would be.

> In other words I think the CXL specification usage of 0x1e98 is scoped
> as "DVSEC Vendor ID", not "Vendor ID".
>
> However that would mean that a future 0x1e98 device could not publish
> DVSECs without colliding with CXL DVSECs.
>
> I note this footnote in section 3.1.2 about the 0x1e98 value (all caps
> emphasis is from the spec, not me):
>
> ---
> NOTICE TO USERS: THE UNIQUE VALUE THAT IS PROVIDED IN THIS CXL SPECIFICATION IS
> FOR USE IN VENDOR DEFINED MESSAGE FIELDS, DESIGNATED VENDOR SPECIFIC EXTENDED
> CAPABILITIES, AND ALTERNATE PROTOCOL NEGOTIATION ONLY...

Right, that's the one I thought was really weird. No sane person
would put crap like that in a spec, which is why I thought there must
be some "interesting" history behind it.

Sigh.