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SubjectRe: [PATCH v1 0/3] arch_topology: Correct CPU capacity scaling
Hi Leo,

On Sunday 13 Mar 2022 at 13:55:09 (+0800), Leo Yan wrote:
> This patch set is to address issues for CPU capacity scaling.
>
> "capacity-dmips-mhz" property might be absent in all CPU nodes, and in
> another situation, DT might have inconsistent binding issue, e.g. some
> CPU nodes have "capacity-dmips-mhz" property and some nodes miss the
> property. Current code mixes these two cases and always rollback to CPU
> capacity 1024 for these two cases.
>
> Patches 01 and 02 in this set are used to distinguish the two different
> DT binding cases, and for the inconsistent binding issue, it rolls back
> to 1024 without CPU capacity scaling.
>
> Patch 03 is to handle the case for absenting "capacity-dmips-mhz"
> property in CPU nodes, the patch proceeds to do CPU capacity scaling based
> on CPU maximum capacity. Thus it can reflect the correct CPU capacity for
> Arm platforms with "fast" and "slow" clusters (CPUs in two clusters have
> the same raw capacity but with different maximum frequencies).
>

In my opinion it's difficult to handle absent "capacity-dmips-mhz"
properties, as they can be a result of 3 scenarios: potential..
1. bug in DT
2. unwillingness to fill this information in DT
3. suggestion that we're dealing with CPUs with same u-arch
(same capacity-dmips-mhz)

I'm not sure it's up to us to interpret suggestions in the code so I
believe treating missing information as error is the right choice, which
is how we're handling this now.

For 3. (and patch 03), isn't it easier to populate capacity-dmips-mhz to
the same value (say 1024) in DT? That is a clear message that we're
dealing with CPUs with the same u-arch.

Thanks,
Ionela.


> This patch set is applied on the mainline kernel with the latest commit
> 68453767131a ("ARM: Spectre-BHB: provide empty stub for non-config").
> And tested on Arm64 Hikey960 platform (with a bit hacking to emulate
> fast and slow clusters).
>
>
> Leo Yan (3):
> arch_topology: Correct semantics for 'cap_parsing_failed'
> arch_topology: Handle inconsistent binding of CPU raw capacity
> arch_topology: Scale CPU capacity if without CPU raw capacity
>
> drivers/base/arch_topology.c | 42 +++++++++++++++++++++++++++++-------
> 1 file changed, 34 insertions(+), 8 deletions(-)
>
> --
> 2.25.1
>
>

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