Re: [PATCH 2/2] PCI/NPEM: Add Native PCIe Enclosure Management support

[Bjorn Helgaas <helgaas@kernel.org>]

On Fri, Mar 22, 2024 at 02:30:03PM -0700, Dan Williams wrote:
> Bjorn Helgaas wrote:
> > On Tue, Mar 12, 2024 at 10:08:16AM +0100, Mariusz Tkaczyk wrote:
> > > On Mon, 11 Mar 2024 17:30:06 -0500
> > > Stuart Hayes <stuart.w.hayes@gmail.com> wrote:
> > > 
> > > > > > No, Linux doesn't support _DSM. It was proposed in previous
> > > > > > iterations by Stuart but I dropped it. We decided that it need to be
> > > > > > strongly rebuild because "pci/pcie" is not right place for ACPI code
> > > > > > so we cannot register _DSM driver instead of NPEM as it was proposed
> > > > > > and I don't have _DSM capable hardware to test it.  
> > > > 
> > > > I'm not sure I understand why pci/pcie isn't the right place for ACPI code--
> > > > there are other _DSMs used in PCI code already, and this _DSM is defined
> > > > in a PCI ECN.
> > > 
> > > I looked into internal review history and I found out that I dropped it after
> > > discussion with Dan Williams:
> > > 
> > > > After review and discussion with Dan _DSM extension is dropped.
> > > 
> > > Unfortunately, I don't remember what exactly he suggested, I just remembered
> > > conclusion that it needs to be reworked and I decided to drop it.
> > > Maybe, I didn't understand him correctly.
> > > 
> > > Dan, could you take a look? Do you remember something?
> > 
> > Straw man proposal:
> > 
> >   - Update this patch so we use NPEM if the device advertises it.
> > 
> >   - If/when Linux support for the _DSM is added, we use the _DSM when
> >     present.  If a device advertises NPEM but no _DSM applies to it,
> >     we use native NPEM for it.
> 
> The current patch matches my last recollection of the discussion, at a
> minimum do not use the NPEM interface when the _DSM is present. That was
> the compromise to meet the spirit of the _DSM definition and leave it to
> folks that care about the _DSM and have hardware to implement and test
> that support.

In the case of an OS that supports native NPEM but not _DSM, I think
it would be unreasonable for NPEM to stop working just because a new
firmware release added the _DSM.

> However, I think the strawman is workable if only because base NPEM
> already has a problem of ambiguity of which NPEM instance in a topology
> should be used.
> 
> For example an NVME or CXL endpoint could have an NPEM implementation
> that is superseded by an NPEM instance in its parent downstream port, or
> another ancestor downstream port / root port.
> 
> The fact that the native NPEM may not be the right interface to use in
> the presence of the _DSM has no specified way to resolve conflicts is at
> least matched by downstream-port vs endpoint conflict resolution not
> being specified.
> 
> So the spec left a bit of a mess and it is reasonable for Linux to say
> "just turn on all the NPEMs and hope that userspace knows what it is
> supposed to do".

If a device and its parent both advertise NPEM, I dunno how to figure
out which to use.  Maybe that's a benefit that could come with the
_DSM.

Bjorn