linux-kernel - Re: [PATCH] ACPI/PPTT: use ACPI ID whenever ACPI_PPTT_ACPI_PROCESSOR_ID

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Date:   Sat, 30 Jun 2018 09:16:20 +0200
From:   Andrew Jones <drjones@...hat.com>
To:     Sudeep Holla <sudeep.holla@....com>
Cc:     linux-acpi@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        Jeremy Linton <jeremy.linton@....com>,
        shunyong.yang@...-semitech.com, yu.zheng@...-semitech.com,
        catalin.marinas@....com, will.deacon@....com,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>
Subject: Re: [PATCH] ACPI/PPTT: use ACPI ID whenever
 ACPI_PPTT_ACPI_PROCESSOR_ID_VALID is set

On Fri, Jun 29, 2018 at 05:17:57PM +0100, Sudeep Holla wrote:
> Currently we use the ACPI processor ID only for the leaf/processor nodes
> as the specification states it must match the value of ACPI processor ID
> field in the processor’s entry in the MADT.
> 
> However, if a PPTT structure represents processors group, it match a
> processor container UID in the namespace and ACPI_PPTT_ACPI_PROCESSOR_ID_VALID
> flag describe whether the ACPI processor ID is valid.
> 
> Lets use UID whenever ACPI_PPTT_ACPI_PROCESSOR_ID_VALID is set to be
> consistent instead of using table offset as it's currently done for non
> leaf nodes.
> 
> Cc: "Rafael J. Wysocki" <rjw@...ysocki.net>
> Signed-off-by: Sudeep Holla <sudeep.holla@....com>
> ---
>  drivers/acpi/pptt.c | 10 ++++++++--
>  1 file changed, 8 insertions(+), 2 deletions(-)
> 
> Hi,
> 
> There's ongoing discussion on assigning ID based in OS using simple
> counters. It can never be consistent with firmware's view. So if the
> firmware provides valid UID for non-processors node, we must use it.

I agree with this. I've been so focused on the fact that the ACPI offsets
are arbitrary, and thus counters can't be worse, that I nearly forgot how
these IDs are actually defined:

>From Documentation/cputopology.txt

"""
1) /sys/devices/system/cpu/cpuX/topology/physical_package_id:

        physical package id of cpuX. Typically corresponds to a physical
        socket number, but the actual value is architecture and platform
        dependent.

2) /sys/devices/system/cpu/cpuX/topology/core_id:

        the CPU core ID of cpuX. Typically it is the hardware platform's
        identifier (rather than the kernel's).  The actual value is
        architecture and platform dependent.
"""

So all my consistency arguments are moot, as no user should expect
consistency from architecture and platform dependent IDs.

A comment on the patch below

> 
> Regards,
> Sudeep
> 
> diff --git a/drivers/acpi/pptt.c b/drivers/acpi/pptt.c
> index e5ea1974d1e3..d1e26cb599bf 100644
> --- a/drivers/acpi/pptt.c
> +++ b/drivers/acpi/pptt.c
> @@ -481,8 +481,14 @@ static int topology_get_acpi_cpu_tag(struct acpi_table_header *table,
>  	if (cpu_node) {
>  		cpu_node = acpi_find_processor_package_id(table, cpu_node,
>  							  level, flag);
> -		/* Only the first level has a guaranteed id */
> -		if (level == 0)
> +		/*
> +		 * As per specification if the processor structure represents
> +		 * an actual processor, then ACPI processor ID must be valid.
> +		 * For processor containers ACPI_PPTT_ACPI_PROCESSOR_ID_VALID
> +		 * should be set if the UID is valid
> +		 */
> +		if (level == 0 ||
> +		    cpu_node->flags & ACPI_PPTT_ACPI_PROCESSOR_ID_VALID)
>  			return cpu_node->acpi_processor_id;
>  		return ACPI_PTR_DIFF(cpu_node, table);
>  	}
> -- 
> 2.7.4
>

When the valid flag is set we'll now return a [hopefully] correct platform
dependent ID, but when it's not we'll return an ACPI table offset. How
will users of the ID know? Also, it's possible to return -ENOENT for the
ID when calling find_acpi_cpu_topology(). How can we distinguish that from
an arbitrary platform dependent ID?

Thanks,
drew