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From:   "Rafael J. Wysocki" <rafael@kernel.org>
Date:   Thu, 14 Dec 2023 18:47:00 +0100
Message-ID: <CAJZ5v0jymOtZ0y65K9wE8FJk+ZKwP+FoGm4AKHXcYVfQJL9MVw@mail.gmail.com>
Subject: Re: [PATCH RFC v3 01/21] ACPI: Only enumerate enabled (or functional) devices
To:     Jonathan Cameron <Jonathan.Cameron@huawei.com>
Cc:     "Russell King (Oracle)" <rmk+kernel@armlinux.org.uk>,
        linux-pm@vger.kernel.org, loongarch@lists.linux.dev,
        linux-acpi@vger.kernel.org, linux-arch@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
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        x86@kernel.org, acpica-devel@lists.linuxfoundation.org,
        linux-csky@vger.kernel.org, linux-doc@vger.kernel.org,
        linux-ia64@vger.kernel.org, linux-parisc@vger.kernel.org,
        Salil Mehta <salil.mehta@huawei.com>,
        Jean-Philippe Brucker <jean-philippe@linaro.org>,
        jianyong.wu@arm.com, justin.he@arm.com,
        James Morse <james.morse@arm.com>
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On Thu, Dec 14, 2023 at 6:32 PM Jonathan Cameron
<Jonathan.Cameron@huawei.com> wrote:
>
> On Wed, 13 Dec 2023 12:49:16 +0000
> Russell King (Oracle) <rmk+kernel@armlinux.org.uk> wrote:
>
> > From: James Morse <james.morse@arm.com>
> >
> > Today the ACPI enumeration code 'visits' all devices that are present.
> >
> > This is a problem for arm64, where CPUs are always present, but not
> > always enabled. When a device-check occurs because the firmware-policy
> > has changed and a CPU is now enabled, the following error occurs:
> > | acpi ACPI0007:48: Enumeration failure
> >
> > This is ultimately because acpi_dev_ready_for_enumeration() returns
> > true for a device that is not enabled. The ACPI Processor driver
> > will not register such CPUs as they are not 'decoding their resources'.
> >
> > Change acpi_dev_ready_for_enumeration() to also check the enabled bit.
> > ACPI allows a device to be functional instead of maintaining the
> > present and enabled bit. Make this behaviour an explicit check with
> > a reference to the spec, and then check the present and enabled bits.
> > This is needed to avoid enumerating present && functional devices that
> > are not enabled.
> >
> > Signed-off-by: James Morse <james.morse@arm.com>
> > Tested-by: Miguel Luis <miguel.luis@oracle.com>
> > Tested-by: Vishnu Pajjuri <vishnu@os.amperecomputing.com>
> > Tested-by: Jianyong Wu <jianyong.wu@arm.com>
> > Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
> > ---
> > If this change causes problems on deployed hardware, I suggest an
> > arch opt-in: ACPI_IGNORE_STA_ENABLED, that causes
> > acpi_dev_ready_for_enumeration() to only check the present bit.
>
> My gut feeling (having made ACPI 'fixes' in the past that ran into
> horribly broken firmware and had to be reverted) is reduce the blast
> radius preemptively from the start. I'd love to live in a world were
> that wasn't necessary but I don't trust all the generators of ACPI tables.
> I'll leave it to Rafael and other ACPI experts suggest how narrow we should
> make it though - arch opt in might be narrow enough.

A chicken bit wouldn't help much IMO, especially in the cases when
working setups get broken.

I would very much prefer to limit the scope of it, say to processors
only, in the first place.