linux-kernel - Re: [PATCH v2 0/5] Cavium ThunderX uncore PMU support

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Date:	Wed, 27 Apr 2016 12:51:56 +0200
From:	Jan Glauber <jan.glauber@...iumnetworks.com>
To:	Will Deacon <will.deacon@....com>
CC:	Mark Rutland <mark.rutland@....com>,
	<linux-kernel@...r.kernel.org>,
	<linux-arm-kernel@...ts.infradead.org>,
	David Daney <ddaney@...iumnetworks.com>
Subject: Re: [PATCH v2 0/5] Cavium ThunderX uncore PMU support

On Tue, Apr 26, 2016 at 02:53:54PM +0100, Will Deacon wrote:

[...]

> > 
> > That sounds like a good compromise.
> > 
> > So I could do the following:
> > 
> > 1) In the uncore setup check for CONFIG_NUMA, if set use the NUMA
> >    information to determine the device node
> > 
> > 2) If CONFIG_NUMA is not set we check if we run on a socketed system
> > 
> >    a) In that case we return an error and give a message that CONFIG_NUMA needs
> >       to be enabled
> >    b) Otherwise we have a single node system and use node_id = 0
> 
> That sounds sensible to me. How do you "check if we run on a socketed
> system"? My assumption would be that you could figure this out from the
> firmware tables?

There are probably multiple ways to detect a socketed system, with some quite
hardware specific. I would like to avoid parsing DT (and ACPI) though,
if possible.

A generic approach would be to do a query of the multiprocessor affinity
register (MPIDR_EL1) on all CPUs. The AFF2 part (bits 23:16) contains the 
socket number on ThunderX. If this is non-zero on any CPU I would assume a
socketed system.

Would that be feasible?

thanks,
Jan