linux-kernel - Re: [PATCH 2/6] cacheinfo: Set cache 'id' based on DT data

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Message-ID: <CAL_JsqKKx5-ep5=FVA5OHM+t=T-9GTuf6Sf9P6ZDUs7RD9=c8g@mail.gmail.com>
Date:   Fri, 17 Dec 2021 12:14:22 -0600
From:   Rob Herring <robh@...nel.org>
To:     Robin Murphy <robin.murphy@....com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        James Morse <james.morse@....com>,
        Jeremy Linton <jeremy.linton@....com>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Sudeep Holla <sudeep.holla@....com>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Len Brown <lenb@...nel.org>, devicetree@...r.kernel.org,
        "open list:ACPI FOR ARM64 (ACPI/arm64)" <linux-acpi@...r.kernel.org>
Subject: Re: [PATCH 2/6] cacheinfo: Set cache 'id' based on DT data

On Fri, Dec 17, 2021 at 10:57 AM Robin Murphy <robin.murphy@....com> wrote:
>
> Hi Rob,
>
> On 2021-12-16 23:31, Rob Herring wrote:
> > Use the minimum CPU h/w id of the CPUs associated with the cache for the
> > cache 'id'. This will provide a stable id value for a given system. As
> > we need to check all possible CPUs, we can't use the shared_cpu_map
> > which is just online CPUs. There's not a cache to CPUs mapping in DT, so
> > we have to walk all CPU nodes and then walk cache levels.
>
> I believe another expected use of the cache ID exposed in sysfs is to
> program steering tags for cache stashing (typically in VFIO-based
> userspace drivers like DPDK so we can't realistically mediate it any
> other way). There were plans afoot last year to ensure that ACPI PPTT
> could provide the necessary ID values for arm64 systems which will
> typically be fairly arbitrary (but unique) due to reflecting underlying
> interconnect routing IDs. Assuming that there will eventually be some
> interest in cache stashing on DT-based systems too, we probably want to
> allow for an explicit ID property on DT cache nodes in a similar manner.

If you have a suggestion for ID values that correspond to the h/w,
then we can add them. I'd like a bit more than just trusting that ID
is something real.

While the ACPI folks may be willing to take an arbitrary index, it's
something we (mostly) avoid for DT.

> That said, I think it does make sense to have some kind of
> auto-generated fallback scheme *as well*, since I'm sure there will be
> plenty systems which care about MPAM but don't support stashing, and
> therefore wouldn't have a meaningful set of IDs to populate their DT
> with. Conversely I think that might also matter for ACPI too - one point
> I remember from previous discussions is that PPTT may use a compact
> representation where a single entry represents all equivalent caches at
> that level, so I'm not sure we can necessarily rely on IDs out of that
> path being unique either.

AIUI, cache ids break the compact representation.

Rob