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Subject: RE: RFC: WMI Enhancements
Thread-Topic: RFC: WMI Enhancements
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Date: Thu, 13 Apr 2017 17:45:08 +0000
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> -----Original Message-----
> From: Andy Lutomirski [mailto:luto@kernel.org]
> Sent: Thursday, April 13, 2017 12:33 PM
> To: Darren Hart <dvhart@infradead.org>
> Cc: Limonciello, Mario <Mario_Limonciello@Dell.com>; Andrew Lutomirski
> <luto@kernel.org>; Michał Kępień <kernel@kempniu.pl>; Rafael J. Wysocki
> <rjw@rjwysocki.net>; Len Brown <len.brown@intel.com>; Pali Rohár
> <pali.rohar@gmail.com>; Corentin Chary <corentin.chary@gmail.com>; Andy
> Shevchenko <andriy.shevchenko@linux.intel.com>; linux-kernel@vger.kernel.org;
> platform-driver-x86@vger.kernel.org; linux-pm@vger.kernel.org
> Subject: Re: RFC: WMI Enhancements
> 
> On Thu, Apr 13, 2017 at 10:02 AM, Darren Hart <dvhart@infradead.org> wrote:
> > On Thu, Apr 13, 2017 at 03:55:01PM +0000, Mario.Limonciello@dell.com wrote:
> >>
> >>
> >> > -----Original Message-----
> >> > From: Andy Lutomirski [mailto:luto@kernel.org]
> >> > Sent: Thursday, April 13, 2017 10:33 AM
> >> > To: Michał Kępień <kernel@kempniu.pl>
> >> > Cc: Darren Hart <dvhart@infradead.org>; Rafael Wysocki
> <rjw@rjwysocki.net>;
> >> > Len Brown <len.brown@intel.com>; Pali Rohár <pali.rohar@gmail.com>;
> Corentin
> >> > Chary <corentin.chary@gmail.com>; Limonciello, Mario
> >> > <Mario_Limonciello@Dell.com>; Andy Lutomirski <luto@kernel.org>; Andy
> >> > Shevchenko <andriy.shevchenko@linux.intel.com>; LKML <linux-
> >> > kernel@vger.kernel.org>; platform-driver-x86@vger.kernel.org; linux-
> >> > pm@vger.kernel.org
> >> > Subject: Re: RFC: WMI Enhancements
> >> >
> >> > On Thu, Apr 13, 2017 at 12:32 AM, Michał Kępień <kernel@kempniu.pl>
> wrote:
> >> > >> Hi All,
> >> > >>
> >> > >> There are a few parallel efforts involving the Windows Management
> >> > >> Instrumentation (WMI)[1] and dependent/related drivers. I'd like to
> >> > >> have a round of discussion among those of you that have been involved
> >> > >> in this space before we decide on a direction.
> >> > >>
> >> > >> The WMI support in the kernel today fairly narrowly supports a
> >> > >> handful of systems. Andy L. has a work-in-progress series [2] which
> >> > >> converts wmi into a platform device and a proper bus, providing
> >> > >> devices for dependent drivers to bind to, and a mechanism for sibling
> devices to
> >> > communicate with each other.
> >> > >> I've reviewed the series and feel like the approach is sound, I plan
> >> > >> to carry this series forward and merge it (with Andy L's permission).
> >> > >>
> >> > >> Are there any objections to this?
> >> > >
> >> > > Back in January 2016, I sent Andy a few minor comments about this
> >> > > series.  A year later, I offered to iron out the remaining issues and
> >> > > resubmit the series in Andy's name when I find the time.  Sadly,
> >> > > things have changed a bit for me since that time and it is unlikely
> >> > > that I will be able to deliver, for which I am sorry.
> >> > >
> >> > > However, browsing Andy's branch I see that most issues have been
> >> > > resolved, though I think some of my remarks [1] have either been
> >> > > missed or silently refuted :)
> >> > >
> >> > > Anyway, I also like this approach and I think this series is a
> >> > > valuable cleanup.
> >> >
> >> > Me too :)
> >> >
> >> > >> In Windows, applications interact with WMI more or less directly. We
> >> > >> don't do this in Linux currently, although it has been discussed in
> >> > >> the past [3]. Some vendors will work around this by performing
> >> > >> SMI/SMM, which is inefficient at best. Exposing WMI methods to
> >> > >> userspace would bring parity to WMI for Linux and Windows.
> >> > >>
> >> > >> There are two principal concerns I'd appreciate your thoughts on:
> >> > >>
> >> > >> a) As an undiscoverable interface (you need to know the method
> >> > >> signatures ahead of time), universally exposing every WMI "device" to
> >> > >> userspace seems like "a bad idea" from a security and stability
> >> > >> perspective. While access would certainly be privileged, it seems
> >> > >> more prudent to make this exposure opt-in. We also handle some of
> >> > >> this with kernel drivers and exposing those "devices" to userspace
> >> > >> would enable userspace and the kernel to fight over control. So - if
> >> > >> we expose WMI devices to userspace, I believe this should be done on
> >> > >> a case by case basis, opting in, and not by default as part of the
> >> > >> WMI driver (although it can provide the mechanism for a sub-driver to use),
> and
> >> > possibly a devmode to do so by default.
> >> >
> >> > I agree.  I don't want too see gnome-whatever-widget talking directly to WMI
> and
> >> > confusing the kernel driver for the same thing.
> >>
> >> So there are plenty of other things that can be done by WMI that don't
> >> really make sense to live in the kernel, particularly on what Dell exposes via
> >> WMI.
> >>
> >> If the desire of this group ends up being to not expose WMI by default,
> >> I'd like to at least propose it be exposed for the GUID's Dell is using.
> >>
> >
> > What I'm thinking is an explicit list of GUIDs within the drivers which are to
> > be exposed to user space. The rationale being:
> >
> > * GUIDs which are managed by kernel drivers (LEDs, hotkeys, etc.) should not be
> >   exposed to userspace.
> >
> > * Management GUIDs should not change frequently
> >
> > * Management GUIDs are a trivial add, equivalent to adding a DEVICE ID to an
> >   existing driver. This means minimal review time to get upstream, and the
> >   ability to include in stable backports as needed. I haven't confirmed
> >   this with Greg KH, but I think I can make the case, especially after
> >   Andy L's WMI-as-a-bus patches.
> 
> Would this be a class driver that would expose a chardev for each
> bound GUID?  I agree that this makes a lot more sense than trying to
> shoehorn it into sysfs.  Especially since we'd want closing the
> chardev to disable any "expensive" collections that have been enabled
> by ioctl on that chardev.  Exposing Dell's smbios entry point through
> this type of device seems reasonable to me.
> 
> If we go this route, then I think that exposing the MOF through sysfs
> would make sense -- after all, someone might actually want to parse
> the thing for production purposes.

I agree.

> 
> On a sort-of-on-topic note, there's one platform feature that we
> complete fail to handle in the kernel that might be nice to add before
> it gets kludged into lots of userspace code: battery charge controls.
> Thinkpads expose charge thresholds using abominable interfaces, but I
> think they've all been reverse-engineered.  Dell probably has them,
> and I bet that Mario would consider telling us how to use them if we
> asked nicely.  It might be nice to expose these generically through
> sysfs somewhere.
> 

Sure.  They're part of the token interface.
https://github.com/dell/libsmbios/blob/master/doc/token_list.csv#L834