linux-kernel - Re: [PATCH] arm64: acpi: reenumerate topology ids

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Date:   Fri, 29 Jun 2018 17:46:08 +0200
From:   Andrew Jones <drjones@...hat.com>
To:     Sudeep Holla <sudeep.holla@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        jeremy.linton@....com, ard.biesheuvel@...aro.org,
        shunyong.yang@...-semitech.com, yu.zheng@...-semitech.com,
        catalin.marinas@....com, will.deacon@....com
Subject: Re: [PATCH] arm64: acpi: reenumerate topology ids

On Fri, Jun 29, 2018 at 02:29:34PM +0100, Sudeep Holla wrote:
> If it matters a lot, vendors must use UID for consistency. Since OS doesn't
> use those IDs for any particular reason, OS must not care.

That depends. If you look at how topology_logical_package_id() is used in
x86 code you'll see it gets used as an index to an array in a couple
places. If we don't remap arbitrary IDs to counters than we may miss out
on some opportunities to avoid lists.

Also, we're talking about what's visible to users. I think it's much more
likely to break a user app by exposing topology IDs that have values
greater than the linear CPU numbers (even though properly written apps
shouldn't expect them to be strictly <=), than the opposite.

> 
> > > 
> > > > >
> > > > > So I would like to keep it simple and just have this counters for
> > > > > package ids as demonstrated in Shunyong's patch.
> > > > >
> > > >
> > > > If we don't also handle cores when there are threads, then the cores
> > > > will also end up having weird IDs.
> > > >
> > > 
> > > Yes, but if PPTT says it has valid ID, I would prefer that over DT like
> > > generated.
> > 
> > Valid *ACPI* ID, which just means it's a guaranteed unique ACPI UID,
> > which isn't likely going to be anything useful to a user.
> > 
> 
> How is that different from OS generated one from user's perspective ?
> Vendors might assign sockets UID and he may help them to replace one.
> Having some generated counter based id is not helpful.

I agree with this. It's a good argument for maintaining a mapping of
package-id to id-physically-printed-on-a-package somewhere. To avoid
maintaining a mapping it could just be stored directly in
cpu_topology[cpu].package_id, but then how can we tell the difference
between a valid printed-on-package-id and an ACPI offset? We'd still
have to maintain additional state to determine if it's valid or not,
so we could just maintain a mapping instead.

Thanks,
drew