linux-kernel - Re: [PATCH v4 2/2] ThunderX2: Add Cavium ThunderX2 SoC UNCORE PMU driver

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Message-ID: <20180521105511.6ztjk5conf7lfaiz@lakrids.cambridge.arm.com>
Date:   Mon, 21 May 2018 11:55:11 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Ganapatrao Kulkarni <gklkml16@...il.com>
Cc:     Ganapatrao Kulkarni <ganapatrao.kulkarni@...ium.com>,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        Will Deacon <Will.Deacon@....com>, jnair@...iumnetworks.com,
        Robert Richter <Robert.Richter@...ium.com>,
        Vadim.Lomovtsev@...ium.com, Jan.Glauber@...ium.com
Subject: Re: [PATCH v4 2/2] ThunderX2: Add Cavium ThunderX2 SoC UNCORE PMU
 driver

On Sat, May 05, 2018 at 12:16:13AM +0530, Ganapatrao Kulkarni wrote:
> On Thu, Apr 26, 2018 at 4:29 PM, Mark Rutland <mark.rutland@....com> wrote:
> > On Wed, Apr 25, 2018 at 02:30:47PM +0530, Ganapatrao Kulkarni wrote:

> >> + *
> >> + *  L3 Tile and DMC channel selection is through SMC call
> >> + *  SMC call arguments,
> >> + *   x0 = THUNDERX2_SMC_CALL_ID      (Vendor SMC call Id)
> >> + *   x1 = THUNDERX2_SMC_SET_CHANNEL  (Id to set DMC/L3C channel)
> >> + *   x2 = Node id
> >
> > How do we map Linux node IDs to the firmware's view of node IDs?
> >
> > I don't believe the two are necessarily the same -- Linux's node IDs are
> > a Linux-specific construct.
> 
> both are same, it is numa node id from ACPI/firmware.

I am very wary about assuming that the Linux nid will always be the same
as the ACPI node id.

For that to *potentially* be true, this driver should depend on
CONFIG_NUMA, NUMA must not be disabled on the command line, etc, or the
node id will always be NUMA_NO_NODE.

I would be *much* happier if we had an explicit mapping somewhere to the
ID the FW expects.

> > It would be much nicer if we could pass something based on the MPIDR,
> > which is a known HW construct, or if this implicitly affected the
> > current node.
> 
> IMO,  node id is sufficient.

I agree that *a* node ID is sufficient, I just don't think that we're
guaranteed to have the specific node ID the FW wants.

> > It would be vastly more sane for this to not be muxed at all. :/
> 
> i am helpless due to crappy hw design!

I'm certainly not blaming you for this! :)

I hope the HW designers don't make the same mistake in future, though...

Thanks,
Mark.