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

[Mariusz Tkaczyk <mariusz.tkaczyk@linux.intel.com>]

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 will follow community voice.

Thanks,
Mariusz