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Date: Mon, 15 Apr 2024 17:50:57 +0100
From: Jonathan Cameron <Jonathan.Cameron@Huawei.com>
To: "Rafael J. Wysocki" <rafael@kernel.org>
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Subject: Re: [PATCH v5 02/18] ACPI: processor: Set the ACPI_COMPANION for
 the struct cpu instance
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On Mon, 15 Apr 2024 18:19:17 +0200
"Rafael J. Wysocki" <rafael@kernel.org> wrote:

> On Mon, Apr 15, 2024 at 6:16=E2=80=AFPM Rafael J. Wysocki <rafael@kernel.=
org> wrote:
> >
> > On Mon, Apr 15, 2024 at 5:49=E2=80=AFPM Jonathan Cameron
> > <Jonathan.Cameron@huawei.com> wrote: =20
> > >
> > > On Fri, 12 Apr 2024 20:10:54 +0200
> > > "Rafael J. Wysocki" <rafael@kernel.org> wrote:
> > > =20
> > > > On Fri, Apr 12, 2024 at 4:38=E2=80=AFPM Jonathan Cameron
> > > > <Jonathan.Cameron@huawei.com> wrote: =20
> > > > >
> > > > > The arm64 specific arch_register_cpu() needs to access the _STA
> > > > > method of the DSDT object so make it available by assigning the
> > > > > appropriate handle to the struct cpu instance.
> > > > >
> > > > > Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
> > > > > ---
> > > > >  drivers/acpi/acpi_processor.c | 3 +++
> > > > >  1 file changed, 3 insertions(+)
> > > > >
> > > > > diff --git a/drivers/acpi/acpi_processor.c b/drivers/acpi/acpi_pr=
ocessor.c
> > > > > index 7a0dd35d62c9..93e029403d05 100644
> > > > > --- a/drivers/acpi/acpi_processor.c
> > > > > +++ b/drivers/acpi/acpi_processor.c
> > > > > @@ -235,6 +235,7 @@ static int acpi_processor_get_info(struct acp=
i_device *device)
> > > > >         union acpi_object object =3D { 0 };
> > > > >         struct acpi_buffer buffer =3D { sizeof(union acpi_object)=
, &object };
> > > > >         struct acpi_processor *pr =3D acpi_driver_data(device);
> > > > > +       struct cpu *c;
> > > > >         int device_declaration =3D 0;
> > > > >         acpi_status status =3D AE_OK;
> > > > >         static int cpu0_initialized;
> > > > > @@ -314,6 +315,8 @@ static int acpi_processor_get_info(struct acp=
i_device *device)
> > > > >                         cpufreq_add_device("acpi-cpufreq");
> > > > >         }
> > > > >
> > > > > +       c =3D &per_cpu(cpu_devices, pr->id);
> > > > > +       ACPI_COMPANION_SET(&c->dev, device); =20
> > > >
> > > > This is also set for per_cpu(cpu_sys_devices, pr->id) in
> > > > acpi_processor_add(), via acpi_bind_one(). =20
> > >
> > > Hi Rafael,
> > >
> > > cpu_sys_devices gets filled with a pointer to this same structure.
> > > The contents gets set in register_cpu() so at this point
> > > it doesn't point anywhere.  As a side note register_cpu()
> > > memsets to zero the value I set it to in the code above which isn't
> > > great, particularly as I want to use this in post_eject for
> > > arm64.
> > >
> > > We could make a copy of the handle and put it back after
> > > the memset in register_cpu() but that is also ugly.
> > > It's the best I've come up with to make sure this is still set
> > > come remove time but is rather odd. =20
> > > >
> > > > Moreover, there is some pr->id validation in acpi_processor_add(), =
so
> > > > it seems premature to use it here this way.
> > > >
> > > > I think that ACPI_COMPANION_SET() should be called from here on
> > > > per_cpu(cpu_sys_devices, pr->id) after validating pr->id (so the
> > > > pr->id validation should all be done here) and then NULL can be pas=
sed
> > > > as acpi_dev to acpi_bind_one() in acpi_processor_add().  Then, there
> > > > will be one physical device corresponding to the processor ACPI dev=
ice
> > > > and no confusion. =20
> > >
> > > I'm fairly sure this is pointing to the same device but agreed this
> > > is a tiny bit confusing. However we can't use cpu_sys_devices at this=
 point
> > > so I'm not immediately seeing a cleaner solution :( =20
> >
> > Well, OK.
> >
> > Please at least consider doing the pr->id validation checks before
> > setting the ACPI companion for &per_cpu(cpu_devices, pr->id).
> >
> > Also, acpi_bind_one() needs to be called on the "physical" devices
> > passed to ACPI_COMPANION_SET() (with NULL as the second argument) for
> > the reference counting and physical device lookup to work.
> >
> > Please also note that acpi_primary_dev_companion() should return
> > per_cpu(cpu_sys_devices, pr->id) for the processor ACPI device, which
> > depends on the order of acpi_bind_one() calls involving the same ACPI
> > device. =20
>=20
> Of course, if the value set by ACPI_COMPANION_SET() is cleared
> subsequently, the above is not needed, but then using
> ACPI_COMPANION_SET() is questionable overall.

Agreed + smoothing over that by stashing and putting it back doesn't
work because there is an additional call to acpi_bind_one() inbetween
here and the one you reference.

The arch_register_cpu() calls end up calling register_cpu() /
device_register() / acpi_device_notify() / acpi_bind_one()

With current code that fails (silently)
If I make sure the handle is set before register_cpu() then it
succeeds, but we end up with duplicate sysfs files etc because we
bind twice.

I think the only way around this is larger reorganization of the
CPU hotplug code to pull the arch_register_cpu() call to where
the acpi_bind_one() call is.  However that changes a lot more than I'd like
(and I don't have it working yet).

Alternatively find somewhere else to stash the handle, or just add it as
a parameter to arch_register_cpu(). Right now this feels the easier
path to me. arch_register_cpu(int cpu, acpi_handle handle)=20

Would that be a path you'd consider?

Jonathan