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@xxxxxxxxx> 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. 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 For example an NVME or CXL endpoint could have an NPEM implementation that is superseded by an NPEM instance in its parent downstream port, or another ancestor downstream port / root port.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".
