Re: [PATCH v2] perf/arm-cmn: Fix multi-filter encoding

[Leo Yan <leo.yan@arm.com>]

On Tue, Sep 15, 2026 at 01:24:22PM +0100, Robin Murphy wrote:
> The current special-case for EVICT_STATE_SEL filtering effectively
> assigns the "filter" and "filter2" controls in the opposite order from
> how the CMN S3 r2 TRM states "Filtering is programmed in pmu_hbt_lbt_sel
> and pmu_evict_state_sel". On reflection, not only does this seem
> unnecessarily non-obvious to users, but it's also likely to be a problem
> for scaling to a full multi-filter abstraction in future. There is a
> logical order to filters based on their bitfield positions in the
> pmu_event_sel register, which the TRM descriptions allude to, and the
> cmn_filter_select enum already (almost) follows, so let's fix the UABI
> to follow suit while it's still unreleased.
> 
> Fixes: 09178f536bb9 ("perf/arm-cmn: Plumb in new filter types")
> Signed-off-by: Robin Murphy <robin.murphy@arm.com>

It's hard to say how useful my review is, but I went through the patch
and also asked AI to explain the rationale. It makes sense to me:

Reviewed-by: Leo Yan <leo.yan@arm.com>

Seems we could implement a generic way to navigate filters based on
the hardware bitfield ordering, e.g. filter_next(sel) and
filter_prev(sel). This can be deferred until we have more combined
filters.

Thanks,
Leo