linux-kernel - Re: [PATCH] powerpc/powernv/pci: remove dead code from !CONFIG

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Message-ID: <CAKwvOdmMugQkTRwC3HOEt2-em2zSfAoi7gpvJRkqfdzSDRMeEg@mail.gmail.com>
Date:   Mon, 17 May 2021 17:16:55 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     "Oliver O'Halloran" <oohall@...il.com>,
        Daniel Axtens <dja@...ens.net>,
        Michael Ellerman <mpe@...erman.id.au>
Cc:     Nathan Chancellor <nathan@...nel.org>,
        Joe Perches <joe@...ches.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Alexey Kardashevskiy <aik@...abs.ru>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] powerpc/powernv/pci: remove dead code from !CONFIG_EEH

On Thu, Apr 22, 2021 at 6:13 PM Oliver O'Halloran <oohall@...il.com> wrote:
>
> On Fri, Apr 23, 2021 at 9:09 AM Daniel Axtens <dja@...ens.net> wrote:
> >
> > Hi Nick,
> >
> > > While looking at -Wundef warnings, the #if CONFIG_EEH stood out as a
> > > possible candidate to convert to #ifdef CONFIG_EEH, but it seems that
> > > based on Kconfig dependencies it's not possible to build this file
> > > without CONFIG_EEH enabled.
> >
> > This seemed odd to me, but I think you're right:
> >
> > arch/powerpc/platforms/Kconfig contains:
> >
> > config EEH
> >         bool
> >         depends on (PPC_POWERNV || PPC_PSERIES) && PCI
> >         default y
> >
> > It's not configurable from e.g. make menuconfig because there's no prompt.
> > You can attempt to explicitly disable it with e.g. `scripts/config -d EEH`
> > but then something like `make oldconfig` will silently re-enable it for
> > you.
> >
> > It's been forced on since commit e49f7a9997c6 ("powerpc/pseries: Rivet
> > CONFIG_EEH for pSeries platform") in 2012 which fixed it for
> > pseries. That moved out from pseries to pseries + powernv later on.
> >
> > There are other cleanups in the same vein that could be made, from the
> > Makefile (which has files only built with CONFIG_EEH) through to other
> > source files. It looks like there's one `#ifdef CONFIG_EEH` in
> > arch/powerpc/platforms/powernv/pci-ioda.c that could be pulled out, for
> > example.
> >
> > I think it's probably worth trying to rip out all of those in one patch?
>
> The change in commit e49f7a9997c6 ("powerpc/pseries: Rivet CONFIG_EEH
> for pSeries platform") never should have been made.

I'll change my patch to keep the conditionals, but use #ifdef instead
of #if then?

>
> There's no inherent reason why EEH needs to be enabled and forcing it
> on is (IMO) a large part of why EEH support is the byzantine
> clusterfuck that it is. One of the things I was working towards was
> allowing pseries and powernv to be built with !CONFIG_EEH since that
> would help define a clearer boundary between what is "eeh support" and
> what is required to support PCI on the platform. Pseries is
> particularly bad for this since PAPR says the RTAS calls needed to do
> a PCI bus reset are part of the EEH extension, but there's non-EEH
> reasons why you might want to use those RTAS calls. The PHB reset that
> we do when entering a kdump kernel is a good example since that uses
> the same RTAS calls, but it has nothing to do with the EEH recovery
> machinery enabled by CONFIG_EEH.
>
> I was looking into that largely because people were considering using
> OPAL for microwatt platforms. Breaking the assumption that
> powernv==EEH support is one of the few bits of work required to enable
> that, but even if you don't go down that road I think everyone would
> be better off if you kept a degree of separation between the two.



-- 
Thanks,
~Nick Desaulniers