Date:   Tue, 09 May 2017 23:24:31 +0200
From:   "Rafael J. Wysocki" <rjw@...ysocki.net>
To:     Darren Hart <dvhart@...radead.org>,
        Michał Kępień <kernel@...pniu.pl>
Cc:     Jonathan Woithe <jwoithe@...t42.net>,
        Andy Shevchenko <andy@...radead.org>,
        platform-driver-x86@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 00/10] fujitsu-laptop: use device-specific data instead of module-wide globals

On Tuesday, May 09, 2017 09:47:34 AM Darren Hart wrote:
> On Tue, May 09, 2017 at 11:35:24AM +0200, Michał Kępień wrote:
> > > On Sat, May 06, 2017 at 02:45:16PM +0200, Michał Kępień wrote:
> > > > > Just to make sure we are all on the same page here, choosing the "two
> > > > > separate modules, each with one driver for one ACPI device" approach
> > > > > would mean ending up with two modules:
> > > > > 
> > > > >   - fujitsu-laptop, binding to the FUJ02E3 ACPI device, handling
> > > > >     everything _except_ backlight,
> > > > > 
> > > > >   - fujitsu-backlight, binding to the FUJ02B1 ACPI device, handling
> > > > >     backlight and depending on fujitsu-laptop.
> > > > > 
> > > > > We would need to export one function from fujitsu-laptop, namely
> > > > > fext_backlight().  I understand this would require creating a separate
> > > > > header file which would then be included in fujitsu-backlight.
> > > > > 
> > > > > fext_backlight() causes the FUNC method of the FUJ02E3 ACPI device to be
> > > > > called.  This method is marked as Serialized, which AFAIU means we do
> > > > > not need a separate lock in kernel code because all calls to this method
> > > > > are implicitly serialized by firmware itself.
> > > > > 
> > > > > I do not see anything "unnatural" in this approach, but I would love to
> > > > > be corrected if I am wrong.
> > > > 
> > > > To be fair, one thing that may be "unnatural" with this approach is that
> > > > even though fujitsu-backlight would depend on fujitsu-laptop, it would
> > > > still have to get a handle to FUJ02E3 using:
> > > > 
> > > >     acpi_get_handle(NULL, "\\_SB.FEXT", ...)
> > > >     
> > > > because call_fext_func() - and thus fext_backlight() - needs to be
> > > > passed a handle to FUJ02E3 and the two ACPI devices (FUJ02B1 handled by
> > > > fujitsu-backlight and FUJ02E3 handled by fujitsu-laptop) are not related
> > > > from the perspective of the ACPI device hierarchy.  Unless there is a
> > > > better way of implementing this, in which case I am open to suggestions.
> > > 
> > > At a high level, I would consider the handle to be private data which should be
> > > encapsulated in fujitsu_laptop. Or... where is FEXT in the ACPI hierarchy
> > > relative to FUJ02E3?
> > 
> > FEXT *is* FUJ02E3:
> > 
> > Device (FEXT)
> > {
> >     Name (_HID, "FUJ02E3")  // _HID: Hardware ID
> >     ...
> >     Method (FUNC, 4, Serialized)
> >     {
> >         ...
> >     }
> >     ...
> > }
> > 
> > See also below.
> > 
> > > Assuming FEXT is below FUJ02E3, the we appear to be making an assumption that
> > > there is only one FUJ02E3 on the system. While I think this is perfectly
> > > reasonable, it does contradict the argumentation from some of the other patches
> > > in this series.
> > 
> > Exactly.  The whole purpose of this patch series is to stop using
> > module-wide data.  We have a different situation here than in the case
> > of e.g. dell-smbios, which coordinates access to a module-wide buffer it
> > allocates.  
> > 
> > > If FEXT is not below fujitsu laptop... then it is a shared function which either
> > > one of them can own and serialize (or not if fw indeed handles that).
> > > 
> > > Either way, the owning driver should abstract away the private data and present
> > > an interface the other can use with only the "public" information.
> > 
> > I feel the problem at hand needs a fresh explanation.  I will be as
> > concise as possible.
> > 
> > We are considering two ACPI devices present on Fujitsu laptops:
> > 
> >   - FJEX:
> >       * path: \_SB_.PCI0.LPCB.FJEX
> >       * HID: FUJ02B1
> >       * methods invoked by kernel: GBLL, RBLL, SBLL, SBL2
> >       * handles: backlight level (LCD brightness)
> > 
> >   - FEXT:
> >       * path: \_SB_.FEXT
> >       * HID: FUJ02E3
> >       * methods invoked by kernel: FUNC
> >       * handles: hotkey, LEDs, platform attributes, backlight power
> >                                                     ^^^^^^^^^^^^^^^
> 
> This is very concise and describes the problem clearly, thank you!
> 
> > 
> > The problem is that if we split the ACPI drivers for those two devices
> > into separate modules, the FJEX driver will need to access the FUNC
> > method of device FEXT, handled by another driver in another module.
> > 
> > One way of solving this cleanly is to store a handle to the most
> > recently found FEXT instance (there should always be at most one anyway)
> > in a module-wide variable inside the FEXT driver, but that defeats the
> > purpose of this series.
> > 
> > Another solution is proposed by patch 04/10 of this series: make the
> > FJEX driver independently grab a handle to FEXT using the absolute ACPI
> > path to the latter.  It feels unnatural (AFAICT only one driver outside
> > drivers/acpi, namely pcc-cpufreq, does that), but it is safe and allows
> > us to drop all module-wide data.
> 
> Rafael's take on this would be useful.

Well, can you point me to patch [04/10] then?

Or better resend the whole series with a CC to linux-acpi (which it should go
to to start with IMO).

Thanks,
Rafael

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