Subject: Re: [PATCH 1/5] powerpc/smp: use cpu_to_chip_id() to find siblings

From: Michael Ellerman <[email-masked]>
Date: Wed, 15 Mar 2017 22:18:22 +1100

Oliver O'Halloran <[email-masked]> writes:

> To determine which logical CPUs are on the same core the kernel uses the
> ibm,chipid property from the device tree node associated with that cpu.
> The lookup for this this information is currently open coded in both
> traverse_siblings() and traverse_siblings_chip_id(). This patch replaces
> these manual lookups with the existing cpu_to_chip_id() function.

Some minor nits.

cpu_to_chip_id() actually searches recursively up the parents until it
finds a ibm,chip-id, so it's not a 1:1 replacement for the existing
logic, but it's probably still an OK conversion. It's still worth
mentioning in the change log thought.

> diff --git a/arch/powerpc/kernel/smp.c b/arch/powerpc/kernel/smp.c
> index 893bd7f79be6..dfe0e1d9cd06 100644
> --- a/arch/powerpc/kernel/smp.c
> +++ b/arch/powerpc/kernel/smp.c
> @@ -664,23 +655,19 @@ static void traverse_core_siblings(int cpu, bool add)
>  {
>  	struct device_node *l2_cache, *np;
>  	const struct cpumask *mask;
> -	int i, chip, plen;
> -	const __be32 *prop;
> +	int chip_id;
> +	int i;
>  
> -	/* First see if we have ibm,chip-id properties in cpu nodes */
> -	np = of_get_cpu_node(cpu, NULL);
> -	if (np) {
> -		chip = -1;
> -		prop = of_get_property(np, "ibm,chip-id", &plen);
> -		if (prop && plen == sizeof(int))
> -			chip = of_read_number(prop, 1);
> -		of_node_put(np);
> -		if (chip >= 0) {
> -			traverse_siblings_chip_id(cpu, add, chip);
> -			return;
> -		}
> +	/* threads that share a chip-id are considered siblings (same die) */

You might know it means the "same die", but AFAIK there's no actual
definition for what the chip-id means, so let's not write comments that
might be wrong in future. Just saying they're considered siblings is
sufficient.

Also "Threads" :)

cheers