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Date: Tue, 23 Jan 2024 10:26:03 +0000
From: Jonathan Cameron <Jonathan.Cameron@Huawei.com>
To: "Russell King (Oracle)" <linux@armlinux.org.uk>
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	<jianyong.wu@arm.com>, <justin.he@arm.com>, James Morse <james.morse@arm.com>
Subject: Re: [PATCH RFC v3 17/21] ACPI: add support to register CPUs based
 on the _STA enabled bit
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On Tue, 2 Jan 2024 14:53:20 +0000
Jonathan Cameron <Jonathan.Cameron@Huawei.com> wrote:

> On Mon, 18 Dec 2023 13:03:32 +0000
> "Russell King (Oracle)" <linux@armlinux.org.uk> wrote:
> 
> > On Wed, Dec 13, 2023 at 12:50:38PM +0000, Russell King wrote:  
> > > From: James Morse <james.morse@arm.com>
> > > 
> > > acpi_processor_get_info() registers all present CPUs. Registering a
> > > CPU is what creates the sysfs entries and triggers the udev
> > > notifications.
> > > 
> > > arm64 virtual machines that support 'virtual cpu hotplug' use the
> > > enabled bit to indicate whether the CPU can be brought online, as
> > > the existing ACPI tables require all hardware to be described and
> > > present.
> > > 
> > > If firmware describes a CPU as present, but disabled, skip the
> > > registration. Such CPUs are present, but can't be brought online for
> > > whatever reason. (e.g. firmware/hypervisor policy).
> > > 
> > > Once firmware sets the enabled bit, the CPU can be registered and
> > > brought online by user-space. Online CPUs, or CPUs that are missing
> > > an _STA method must always be registered.    
> > 
> > ...
> >   
> > > @@ -526,6 +552,9 @@ static void acpi_processor_post_eject(struct acpi_device *device)
> > >  		acpi_processor_make_not_present(device);
> > >  		return;
> > >  	}
> > > +
> > > +	if (cpu_present(pr->id) && !(sta & ACPI_STA_DEVICE_ENABLED))
> > > +		arch_unregister_cpu(pr->id);    
> > 
> > This change isn't described in the commit log, but seems to be the cause
> > of the build error identified by the kernel build bot that is fixed
> > later in this series. I'm wondering whether this should be in a
> > different patch, maybe "ACPI: Check _STA present bit before making CPUs
> > not present" ?  
> 
> Would seem a bit odd to call arch_unregister_cpu() way before the code
> is added to call the matching arch_registers_cpu()
> 
> Mind you this eject doesn't just apply to those CPUs that are registered
> later I think, but instead to all.  So we run into the spec hole that
> there is no way to identify initially 'enabled' CPUs that might be disabled
> later.
> 
> > 
> > Or maybe my brain isn't working properly (due to being Covid positive.)
> > Any thoughts, Jonathan?  
> 
> I'll go with a resounding 'not sure' on where this change belongs.
> I blame my non existent start of the year hangover.
> Hope you have recovered!

Looking again, I think you were right, move it to that earlier patch.

J
> 
> Jonathan
> 
> 
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