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@xxxxxxxxxx> 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
