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

[Bjorn Helgaas <helgaas@kernel.org>]

On Mon, Mar 11, 2024 at 10:47:50AM +0100, Mariusz Tkaczyk wrote:
> On Wed, 6 Mar 2024 16:40:08 -0600
> Bjorn Helgaas <helgaas@kernel.org> wrote:
>  
> > > +	pos = pci_find_ext_capability(dev, PCI_EXT_CAP_ID_NPEM);
> > > +	if (pos == 0)
> > > +		return;
> > > +
> > > +	if (pci_read_config_dword(dev, pos + PCI_NPEM_CAP, &cap) != 0 ||
> > > +	    (cap & PCI_NPEM_CAPABLE) == 0)
> > > +		return;
> > > +
> > > +	/*
> > > +	 * OS should use the DSM for LED control if it is available
> > > +	 * PCI Firmware Spec r3.3 sec 4.7.
> > > +	 */
> > > +	if (npem_has_dsm(dev))
> > > +		return;  
> > 
> > Does Linux have support for this _DSM?  I don't see any, and I guess
> > this check means that if we have a device that supports NPEM on a
> > platform that supplies this _DSM, we can't use NPEM.
> 
> 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.
> 
> > The stated intent of the _DSM (from the Feb 12, 2020 ECN at
> > https://members.pcisig.com/wg/PCI-SIG/document/14025) is to provide
> > NPEM-like functionality via an abstraction layer on top of NPEM, SES,
> > or other implementations.
> > 
> > The _DSM also gives the platform a chance to intercept and change or
> > reject indications requested by OSPM, although that isn't mentioned as
> > part of the intent.
> > 
> > I'm interested in your thoughts about this.  One possibility would be
> > to omit this check for now and add it back when the _DSM is supported,
> > so we could use NPEM directly when advertised by a device.  Or we
> > could keep this as-is and prohibit use of NPEM if the _DSM exists,
> > even though we know how to operate it.
> 
> I decided to implement it 2nd way because I don't know if I can use
> NPEM if _DSM is implemented, I mean that hardware may do not
> response on NPEM requests.  I choose safer approach, I have no
> opinion.

I think your point is that if the _DSM is supported, the platform
itself might be using NPEM internally, and maybe that would conflict
with an OS that uses NPEM directly, which is a good question.

There is no ownership negotiation, e.g., via _OSC, so my assumption is
that the OS owns NPEM by default, and the platform should not touch a
PCI device to operate NPEM after booting the OS.  I guess the platform
must take ownership of the NPEM Capability if the OS uses the _DSM,
although the spec isn't very explicit about this.

One concern here is that if the OS avoids use of NPEM when the _DSM is
present, NPEM will work on the OS we ship today (with NPEM but no _DSM
support), but it will break as soon as a new platform or new firmware
release adds the _DSM, and users will have no way to fix it.

Bjorn