linux-kernel - Re: [PATCH RFC v3 01/21] ACPI: Only enumerate enabled (or functional) devices

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Message-ID: <ZXtBgtB8FXs8ge0h@shell.armlinux.org.uk>
Date:   Thu, 14 Dec 2023 17:55:14 +0000
From:   "Russell King (Oracle)" <linux@...linux.org.uk>
To:     Jonathan Cameron <Jonathan.Cameron@...wei.com>
Cc:     linux-pm@...r.kernel.org, loongarch@...ts.linux.dev,
        linux-acpi@...r.kernel.org, linux-arch@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-riscv@...ts.infradead.org, kvmarm@...ts.linux.dev,
        x86@...nel.org, acpica-devel@...ts.linuxfoundation.org,
        linux-csky@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-ia64@...r.kernel.org, linux-parisc@...r.kernel.org,
        Salil Mehta <salil.mehta@...wei.com>,
        Jean-Philippe Brucker <jean-philippe@...aro.org>,
        jianyong.wu@....com, justin.he@....com,
        James Morse <james.morse@....com>
Subject: Re: [PATCH RFC v3 01/21] ACPI: Only enumerate enabled (or
 functional) devices

On Thu, Dec 14, 2023 at 05:32:41PM +0000, Jonathan Cameron wrote:
> On Wed, 13 Dec 2023 12:49:16 +0000
> Russell King (Oracle) <rmk+kernel@...linux.org.uk> wrote:
> 
> > From: James Morse <james.morse@....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@....com>
> > Tested-by: Miguel Luis <miguel.luis@...cle.com>
> > Tested-by: Vishnu Pajjuri <vishnu@...amperecomputing.com>
> > Tested-by: Jianyong Wu <jianyong.wu@....com>
> > Signed-off-by: Russell King (Oracle) <rmk+kernel@...linux.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.

Yes, I think an arch opt-in would be the most sensible way forward, if
Rafael concurs with that idea. I notice that what I wrote there was
actually an opt-out. I'll fix that.

> > +	/*
> > +	 * ACPI 6.5's 6.3.7 "_STA (Device Status)" allows firmware to return
> > +	 * (!present && functional) for certain types of devices that should be
> > +	 * enumerated. Note that the enabled bit can't be sert until the present
> 
> set until

Thanks for spotting that, fixed.

-- 
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