Re: [PATCH v6 1/2] of: Documentation: Specify local APIC ID in "reg"

From: Ivan Gorinov
Date: Tue Mar 20 2018 - 14:18:18 EST


On Mon, Mar 19, 2018 at 07:39:52PM -0500, Rob Herring wrote:
> On Tue, Mar 13, 2018 at 5:05 PM, Ivan Gorinov <ivan.gorinov@xxxxxxxxx> wrote:
> > Use the "reg" property to specify the processor's local APIC ID.
> > Local APIC ID is assigned by hardware and may differ from CPU number.
>
> Is "CPU number" a s/w visible h/w number or has it just been an index
> for DT? In the latter case, I'm okay with this change. In the former
> case, you should stick to the existing numbering. For example on ARM,
> the number here corresponds to a core ID number in a register called
> MPIDR.

The latter case. Apparently, "CPU number" was just an index in the list.
Local APIC ID is the s/w visible h/w assigned number.
Some processor models allow local APIC ID to be changed by software, but
CPUID instruction executed with %eax = 0x0b always returns the initial ID
assigned by hardware in %edx.

APIC ID does not match index in the list in many systems.

> >
> > Signed-off-by: Ivan Gorinov <ivan.gorinov@xxxxxxxxx>
> > ---
> > Documentation/devicetree/bindings/x86/ce4100.txt | 37 ++++++++++++++++++------
> > 1 file changed, 28 insertions(+), 9 deletions(-)
> >
> > diff --git a/Documentation/devicetree/bindings/x86/ce4100.txt b/Documentation/devicetree/bindings/x86/ce4100.txt
> > index b49ae59..1c41cbd 100644
> > --- a/Documentation/devicetree/bindings/x86/ce4100.txt
> > +++ b/Documentation/devicetree/bindings/x86/ce4100.txt
> > @@ -7,17 +7,36 @@ Many of the "generic" devices like HPET or IO APIC have the ce4100
> > name in their compatible property because they first appeared in this
> > SoC.
> >
> > -The CPU node
> > -------------
> > - cpu@0 {
> > - device_type = "cpu";
> > - compatible = "intel,ce4100";
> > - reg = <0>;
> > - lapic = <&lapic0>;
> > +The CPU nodes
> > +-------------
> > +
> > + cpus {
> > + #address-cells = <1>;
> > + #size-cells = <0>;
> > +
> > + cpu@0x00 {
>
> Drop the '0x' and leading 0s.
>
> > + device_type = "cpu";
> > + compatible = "intel,ce4100";
> > + reg = <0x00>;
> > + };
> > +
> > + cpu@0x02 {
> > + device_type = "cpu";
> > + compatible = "intel,ce4100";
> > + reg = <0x02>;
> > + };
> > };
> >
> > -The reg property describes the CPU number. The lapic property points to
> > -the local APIC timer.
> > +A "cpu" node describes one logical processor (hardware thread).
> > +
> > +Required properties:
> > +
> > +- device_type
> > + Device type, must be "cpu".
> > +
> > +- reg
> > + Local APIC ID, the unique number assigned to each processor by
> > + system hardware.
> >
> > The SoC node
> > ------------
> > --
> > 2.7.4
> >