Re: [PATCH v2 02/20] libnd, nd_acpi: initial libnd infrastructure and NFIT support

Rafael J. Wysocki
2015-05-04T23:33:15Z

On Friday, May 01, 2015 09:23:38 AM Dan Williams wrote:
> On Thu, Apr 30, 2015 at 6:21 PM, Rafael J. Wysocki <rjw@rjwysocki.net> wrote:
> > On Thursday, April 30, 2015 05:39:06 PM Dan Williams wrote:
> >> On Thu, Apr 30, 2015 at 4:23 PM, Rafael J. Wysocki <rjw@rjwysocki.net> wrote:
> [..]
> >> >> +if ND_DEVICES
> >> >> +
> >> >> +config LIBND
> >> >> + tristate "LIBND: libnd device driver support"
> >> >> + help
> >> >> + Platform agnostic device model for a libnd bus. Publishes
> >> >> + resources for a PMEM (persistent-memory) driver and/or BLK
> >> >> + (sliding mmio window(s)) driver to attach. Exposes a device
> >> >> + topology under a "ndX" bus device, a "/dev/ndctlX" bus-ioctl
> >> >> + message passing interface, and a "/dev/nmemX" dimm-ioctl
> >> >> + message interface for each memory device registered on the
> >> >> + bus. instance. A userspace library "ndctl" provides an API
> >> >> + to enumerate/manage this subsystem.
> >> >> +
> >> >> +config ND_ACPI
> >> >> + tristate "ACPI: NFIT to libnd bus support"
> >> >> + select LIBND
> >> >> + depends on ACPI
> >> >> + help
> >> >> + Infrastructure to probe ACPI 6 compliant platforms for
> >> >> + NVDIMMs (NFIT) and register a libnd device tree. In
> >> >> + addition to storage devices this also enables libnd craft
> >> >> + ACPI._DSM messages for platform/dimm configuration.
> >> >
> >> > I'm wondering if the two CONFIG options above really need to be user-selectable?
> >> >
> >> > For example, what reason people (who've already selected ND_DEVICES) may have
> >> > for not selecting ND_ACPI if ACPI is set?
> >>
> >>
> >> Later on in the series we introduce ND_E820 which supports creating a
> >> libnd-bus from e820-type-12 memory ranges on pre-NFIT systems. I'm
> >> also considering a configfs defined libnd-bus because e820 types are
> >> not nearly enough information to safely define nvdimm resources
> >> outside of NFIT.
> >
> > I hope these are not mutually exclusive with ND_ACPI? Otherwise distros
> > will have problems with supporting them in one kernel.
>
> You can have ND_E820 support and ND_ACPI support in the same system.
> Likely an NFIT enabled system will never have e820-type-12 ranges, but
> if a user messes up and uses the new memmap=ss!nn command line to
> overlap NFIT-defined memory then the request_mem_region() calls in the
> driver will collide. First to load wins in that scenario.
>
> > If ND_E820 and ND_ACPI aren't mutually exclusive, I still don't see a good
> > enough reason for asking users about ND_ACPI. Why would I ever say "No"
> > here if I said "Yes" or "Module" to ND_DEVICES?
>
> I agree that if the user selects ND_DEVICES then ND_ACPI should
> probably default on, but otherwise turning it off is a useful option.
> If you know your system is pre-ACPI-6 then why bother including
> support?

If you're a distro, you don't care. You have to support it regardless.

You might care if you're an end user building a kernel for yourself and just
for this particular specific machine. Honestly, how many *server* users do
that?

And fewer user-selectable options means fewer combination of options to test
during development/validation.

Also unrelated, but applies to this patch.

Since your new driver will handle device ID ACPI0012 which is defined by the
spec proper, it should go into drivers/acpi/, because there's where such things
go as a rule.

--
I speak only for myself.
Rafael J. Wysocki, Intel Open Source Technology Center.