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From:   Pedro Falcato <pedro.falcato@gmail.com>
Date:   Thu, 2 Feb 2023 15:48:02 +0000
Message-ID: <CAKbZUD3UPMuH-NHXL093FaaFtEaeC295RemSeVMC8jLr6zbghA@mail.gmail.com>
Subject: Re: [PATCH] ACPI: Make custom_method use per-open state
To:     "Rafael J. Wysocki" <rafael@kernel.org>
Cc:     "Marty E. Plummer" <hanetzer@startmail.com>,
        Sebastian Grzywna <swiftgeek@gmail.com>,
        linux-acpi@vger.kernel.org, Len Brown <lenb@kernel.org>,
        linux-kernel@vger.kernel.org, rui.zhang@intel.com,
        Hang Zhang <zh.nvgt@gmail.com>
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On Thu, Feb 2, 2023 at 10:45 AM Rafael J. Wysocki <rafael@kernel.org> wrote=
:
>
> On Thu, Feb 2, 2023 at 11:03 AM Rafael J. Wysocki <rafael@kernel.org> wro=
te:
> >
> > On Thu, Feb 2, 2023 at 8:50 AM Sebastian Grzywna <swiftgeek@gmail.com> =
wrote:
> > >
> > > Dnia 2023-02-01, o godz. 19:34:48
> > > "Rafael J. Wysocki" <rafael@kernel.org> napisa=C5=82(a):
> > >
> > > > On Wed, Feb 1, 2023 at 12:38 AM Pedro Falcato
> > > > <pedro.falcato@gmail.com> wrote:
> > > > >
> > > > > Make custom_method keep its own per-file-open state instead of
> > > > > global state in order to avoid race conditions[1] and other
> > > > > possible conflicts with other concurrent users.
> > > > >
> > > > > Link:
> > > > > https://lore.kernel.org/linux-acpi/20221227063335.61474-1-zh.nvgt=
@gmail.com/
> > > > > # [1] Reported-by: Hang Zhang <zh.nvgt@gmail.com> Cc: Swift Geek
> > > > > <swiftgeek@gmail.com> Signed-off-by: Pedro Falcato
> > > > > <pedro.falcato@gmail.com> ---
> > > > >  This patch addresses Hang's problems plus the ones raised by
> > > > > Rafael in his review (see link above).
> > > > > https://lore.kernel.org/lkml/2667007.mvXUDI8C0e@kreacher/ was
> > > > > submitted but since there were still people that wanted this
> > > > > feature, I took my time to write up a patch that should fix the
> > > > > issues. Hopefully the linux-acpi maintainers have not decided to
> > > > > remove custom_method just yet.
> > > >
> > > > Well, thanks for the patch, but yes, they have.  Sorry.
> > >
> > > Hi Rafael,
> > > Can you please explain why you don't want to keep it, given there's a
> > > patch?
> >
> > Because this interface was a bad idea to start with and its
> > implementation is questionable at the design level.
> >
> > Granted, at the time it was introduced, there was no alternative, but
> > there is the AML debugger in the kernel now and as far as debugging is
> > concerned, it is actually more powerful than custom_metod AFAICS.  See
> > Documentation/firmware-guide/acpi/aml-debugger.rst.
> >
> > If the AML debugger has problems, I would very much prefer fixing them
> > to the perpetual maintenance of custom_method.
> >
> > > I find it really useful in my day-to-day as a firmware engineer.
> > > I don't see much happening in git history of
> > > drivers/acpi/custom_method.c , and I don't see anything that was
> > > specifically changed in it in past 10 years to keep it being
> > > functional. Without your more detailed explanation I have hard time
> > > understanding your decision to remove it, since I'm not a kernel
> > > developer myself.
> >
> > It's been always conceptually questionable, problematic from the
> > security standpoint and implemented poorly.  Also its documentation is
> > outdated.
> >
> > The patches fixing its most apparent functional issues don't actually
> > address much of the above.
> >
> > The AML debugger should really be used for debug rather than
> > custom_method and honestly, what's the purpose of it beyond debug?
>
> The above said, if people really do care about custom_method, it can
> be retained, but its documentation needs to be updated to cover the
> current requirements (according to Rui, they have changed after some
> upstream ACPICA changes).
>
> Also note that the upstream ACPICA may not be guaranteed to avoid
> breaking this interface in the future, as it depends on
> acpi_install_method() that is provided by ACPICA specifically for the
> use in the AML debugger.

Just to be clear, I have no stake in this matter and wrote this patch becau=
se
someone complained on #coreboot about the removal. This patch was mostly
trivial to write and if you decide there really is no value in keeping
this, I'm a-OK.

From a maintainers' PoV, if the AML debugger completely encompasses this
from a functional PoV with a good UX, I would probably encourage you to
remove this once and for all (what's the value in having 2 of the same
functionality?).

Hopefully the FW folks can give more context and feedback.

--=20
Pedro