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

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Message-ID: <20180629155556.GD16282@e107155-lin>
Date:   Fri, 29 Jun 2018 16:55:56 +0100
From:   Sudeep Holla <sudeep.holla@....com>
To:     Andrew Jones <drjones@...hat.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,
        Sudeep Holla <sudeep.holla@....com>
Subject: Re: [PATCH] arm64: acpi: reenumerate topology ids

On Fri, Jun 29, 2018 at 05:46:08PM +0200, Andrew Jones wrote:
> On Fri, Jun 29, 2018 at 02:29:34PM +0100, Sudeep Holla wrote:

[..]

> > 
> > 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.
> 

x86 may have a architectural way to obtain it and hence they don't need
to rely on PPTT. But for ARM, we need to rely on PPTT for it and if
vendors/users need accurate information, it has to come from PPTT and
any other place is never going to be consistent and hence unusable.
So, even though specification doesn't mandate, I think OS should as it's
the only robust way. We can get the firmware fixed/updated if random
unique number hurts. Firmware is not upgradeable is no longer a valid
argument.

-- 
Regards,
Sudeep