Re: [PATCH RFC v4 02/15] ACPI: processor: Register all CPUs from acpi_processor_get_info()

From: Jonathan Cameron <Jonathan.Cameron@Huawei.com>
To: "Rafael J. Wysocki" <rafael@kernel.org>
Cc: "Russell King (Oracle)" <linux@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>,
	<linux-riscv@lists.infradead.org>, <kvmarm@lists.linux.dev>,
	<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>,
	Miguel Luis <miguel.luis@oracle.com>
Subject: Re: [PATCH RFC v4 02/15] ACPI: processor: Register all CPUs from acpi_processor_get_info()
Date: Wed, 10 Apr 2024 22:07:12 +0100	[thread overview]
Message-ID: <20240410220712.0000726f@Huawei.com> (raw)
In-Reply-To: <CAJZ5v0gG0xLajHsWXVM+-V+fQZAudvojechUa-DzFgwCs2q8Dg@mail.gmail.com>

On Wed, 10 Apr 2024 21:08:06 +0200
"Rafael J. Wysocki" <rafael@kernel.org> wrote:

> On Wed, Apr 10, 2024 at 8:56 PM Russell King (Oracle)
> <linux@armlinux.org.uk> wrote:
> >
> > On Wed, Apr 10, 2024 at 02:50:05PM +0100, Jonathan Cameron wrote:  
> > > If we get rid of this catch all, solution would be to move the
> > > !acpi_disabled check into the arm64 version of arch_cpu_register()
> > > because we would only want the delayed registration path to be
> > > used on ACPI cases where the question of CPU availability can't
> > > yet be resolved.  
> >
> > Aren't we then needing two arch_register_cpu() implementations?
> > I'm assuming that you're suggesting that the !acpi_disabled, then
> > do nothing check is moved into arch_register_cpu() - or to put it
> > another way, arch_register_cpu() does nothing if ACPI is enabled.
> >
> > If arch_register_cpu() does nothing if ACPI is enabled, how do
> > CPUs get registered (and sysfs files get created to control them)
> > on ACPI systems? ACPI wouldn't be able to call arch_register_cpu(),
> > so I suspect you'll need an ACPI-specific version of this function.  
> 
> arch_register_cpu() will do what it does, but it will check (upfront)
> if ACPI is enabled and if so, if the ACPI Namespace is available.  In
> the case when ACPI is enabled and the ACPI Namespace is not ready, it
> will return -EPROBE_DEFER (say).

Exactly.  I oversimplified and wasn't clear enough.
The check is there in the arch_register_cpu() and is one of the ways
that function can decide to actually register the cpu but not the only one.

I think we may later want to consider breaking it into 2 arch calls
(check if ready to register + register) to reduce code duplication
in with the hotplug path where there is a little extra to do
inbetween.

Hopefully that can wait though.

Jonathan