Re: [PATCH 1/1] arm: topology: parse the topology from the dt

[Ruifeng Zhang <ruifeng.zhang0110@gmail.com>]

Thanks for your review. Patch-v2 that solve the capacity issue will be
uploaded as soon as possible. : )

Valentin Schneider <valentin.schneider@arm.com> 于2021年4月13日周二 下午7:40写道：
>
> On 13/04/21 14:13, Ruifeng Zhang wrote:
> > Valentin Schneider <valentin.schneider@arm.com> 于2021年4月12日周一 下午11:33写道：
> >> I'm not fluent at all in armv7 (or most aarch32 compat mode stuff), but
> >> I couldn't find anything about MPIDR format differences:
> >>
> >>   DDI 0487G.a G8.2.113
> >>   """
> >>   AArch32 System register MPIDR bits [31:0] are architecturally mapped to
> >>   AArch64 System register MPIDR_EL1[31:0].
> >>   """
> >>
> >> Peeking at some armv7 doc and arm/kernel/topology.c the layout really looks
> >> just the same, i.e. for both of them, with your example of:
> >
> > The cortex-a7 spec DDI0464F 4.3.5
> > https://developer.arm.com/documentation/ddi0464/f/?lang=en
> >
>
> Ah, so that's where the core_id=bit[1:0] comes from. That does still
> conform to the MPIDR format, and as you point out below that's being parsed
> the same (aff2, aff1, aff0) == mpidr([23:16][15:8][7:0])
>
> > The current arch/arm/kernel/topology code parse the MPIDR with a armv7 format.
> > the parse code is:
> > void store_cpu_topology(unsigned int cpuid)
> > {
> >     ...
> >     cpuid_topo->thread_id = -1;
> >     cpuid_topo->core_id = MPIDR_AFFINITY_LEVEL(mpidr, 0);
> >     cpuid_topo->package_id = MPIDR_AFFINITY_LEVEL(mpidr, 1);
> >     ...
> > }
> >>
> >>   core0: 0000000080000000
> >>   core1: 0000000080000100
> >>   core2: 0000000080000200
> >>   ...
> >>
> >> we'll get:
> >>
> >>   |       | aff2 | aff1 | aff0 |
> >>   |-------+------+------+------|
> >>   | Core0 |    0 |    0 |    0 |
> >>   | Core1 |    0 |    1 |    0 |
> >>   | Core2 |    0 |    2 |    0 |
> >>       ...
> >>
> >> Now, arm64 doesn't fallback to MPIDR for topology information anymore since
> >>
> >>   3102bc0e6ac7 ("arm64: topology: Stop using MPIDR for topology information")
> >>
> >> so without DT we would get:
> >>   |       | package_id | core_id |
> >>   |-------+------------+---------|
> >>   | Core0 |          0 |       0 |
> >>   | Core1 |          0 |       1 |
> >>   | Core2 |          0 |       2 |
> >>
> >> Whereas with an arm kernel we'll end up parsing MPIDR as:
> >>   |       | package_id | core_id |
> >>   |-------+------------+---------|
> >>   | Core0 |          0 |       0 |
> >>   | Core1 |          1 |       0 |
> >>   | Core2 |          2 |       0 |
> >>
> >> Did I get this right? Is this what you're observing?
> >
> > Yes, this is a problem if an armv8.2 or above cpu is running a 32-bit
> > kernel on EL1.
>
>
> With the above MPIDR(_EL1) values, you would have the same problem in
> aarch64 mode on any kernel predating
>
>   3102bc0e6ac7 ("arm64: topology: Stop using MPIDR for topology information")
>
> since all Aff0 values are 0. Arguably those MPIDR(_EL1) values don't
> make much sense (cores in the same cluster should have different Aff0
> values, unless SMT), but in arm64 that's usually "corrected" by DT.
>
> As you pointed out, arm doesn't currently leverage the cpu-map DT entry. I
> don't see any obvious problem with adding support for it, so if you can fix
> the capacity issue Dietmar reported, I think we could consider it.