Message-ID: <20170509093524.GA19713@ozzy.nask.waw.pl>
Date:   Tue, 9 May 2017 11:35:24 +0200
From:   Michał Kępień <kernel@...pniu.pl>
To:     Darren Hart <dvhart@...radead.org>
Cc:     Jonathan Woithe <jwoithe@...t42.net>,
        Rafael Wysocki <rjw@...ysocki.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 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
                                                    ^^^^^^^^^^^^^^^

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.

Finally, perhaps the approach I took in my patch series is simply too
zealous.  Maybe the simplest solution is to just keep using module-wide
data, but then we are left with a single module with two intertwined
ACPI drivers inside that need to be registered in the correct order.  It
feels a bit brittle.

> I suggest investigating the various mechanisms Andy pointed at and revisiting
> this after that.

For now, these yield no immediate silver bullets for me.  But I will dig
a bit deeper and report back.  Meanwhile, if anyone feels like sharing
their thoughts after reading the summary I wrote above, I am all ears.

-- 
Best regards,
Michał Kępień

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