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

[Rob Herring <robh@kernel.org>]

On Fri, Dec 17, 2021 at 1:03 PM Sudeep Holla <sudeep.holla@arm.com> wrote:
>
> On Fri, Dec 17, 2021 at 12:14:22PM -0600, Rob Herring wrote:
> > On Fri, Dec 17, 2021 at 10:57 AM Robin Murphy <robin.murphy@arm.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
>
> I am trying to follow the code. IIUC, the level one(I$ and D$) are skipped
> in this logic and the private unified cache if any will get the cpu hwid as
> the cache id which is all fine. But what happens if there are 2 levels of
> unified private cache ? I am assuming we only care about shared caches for
> MPAM and ignore private caches which sounds OK but I just wanted to confirm.

The cacheinfo 'id' is only unique to the level and type. It's the
type, level, and ID that gives a unique identifier:

 * struct cacheinfo - represent a cache leaf node
 * @id: This cache's id. It is unique among caches with the same (type, level).

Maybe ACPI's ID expects/allows globally unique cache IDs?

> > > > 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 would have preferred to add the cache IDs in DT similar to ACPI but I see
> you have certain concerns with that which are valid as well.
>
> > >
> > > 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.
> >
>
> I agree, probably architecture must do better job at defining these. But
> generated IDs IMO might cause issues especial if we have to change the
> logic without breaking the backward compatibility.
>
> > While the ACPI folks may be willing to take an arbitrary index, it's
> > something we (mostly) avoid for DT.
> >
>
> Not sure if we can call that *arbitrary* 😄, in that case we can imagine
> the same at several places in the firmware.

By arbitrary, I mean made up by the binding/dts author or
documentation convention (UART0, UART1, etc.). Certainly things like
clock IDs are often made up number spaces, but I don't see how we
avoid that. DT had 'cell-index' which I still see attempted. But that
property traces back to h/w having a single power ctrl register and
cell-index was the bit index for the register. If only h/w was still
that simple.

Rob