linux-kernel - Re: [PATCH 1/2] powerpc: Detect the presence of big-core with interleaved threads

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Date:   Tue, 22 May 2018 10:04:30 +0530
From:   Gautham R Shenoy <ego@...ux.vnet.ibm.com>
To:     Michael Ellerman <mpe@...erman.id.au>
Cc:     "Gautham R. Shenoy" <ego@...ux.vnet.ibm.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Michael Neuling <mikey@...ling.org>,
        Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>,
        Akshay Adiga <akshay.adiga@...ux.vnet.ibm.com>,
        Shilpasri G Bhat <shilpa.bhat@...ux.vnet.ibm.com>,
        Balbir Singh <bsingharora@...il.com>,
        "Oliver O'Halloran" <oohall@...il.com>,
        Nicholas Piggin <npiggin@...il.com>,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] powerpc: Detect the presence of big-core with
 interleaved threads

Hello Michael,

On Fri, May 18, 2018 at 11:21:22PM +1000, Michael Ellerman wrote:
> "Gautham R. Shenoy" <ego@...ux.vnet.ibm.com> writes:
> 
> > diff --git a/arch/powerpc/kernel/setup-common.c b/arch/powerpc/kernel/setup-common.c
> > index 0af5c11..884dff2 100644
> > --- a/arch/powerpc/kernel/setup-common.c
> > +++ b/arch/powerpc/kernel/setup-common.c
> > @@ -436,8 +438,56 @@ static void __init cpu_init_thread_core_maps(int tpc)
> >  	printk(KERN_DEBUG " (thread shift is %d)\n", threads_shift);
> >  }
> >  
> > -
> >  u32 *cpu_to_phys_id = NULL;
> > +/*
> > + * check_for_interleaved_big_core - Checks if the core represented by
> > + *	 dn is a big-core whose threads are interleavings of the
> > + *	 threads of the component small cores.
> > + *
> > + * @dn: device node corresponding to the core.
> > + *
> > + * Returns true if the core is a interleaved big-core.
> > + * Returns false otherwise.
> > + */
> > +static inline bool check_for_interleaved_big_core(struct device_node *dn)
> > +{
> > +	int len, nr_groups, threads_per_group;
> > +	const __be32 *thread_groups;
> > +	__be32 *thread_list, *first_cpu_idx;
> > +	int cur_cpu, next_cpu, i, j;
> > +
> > +	thread_groups = of_get_property(dn, "ibm,thread-groups", &len);
> > +	if (!thread_groups)
> > +		return false;
> 
> There are better device tree APIs than bare of_get_property() these
> days, can you try to use those?

Ok, I will use them.

> 
> > +	nr_groups = be32_to_cpu(*(thread_groups + 1));
> > +	if (nr_groups <= 1)
> > +		return false;
> 
> eg. this would be of_property_read_u32_index()
> 

Ok.

> > @@ -565,7 +615,16 @@ void __init smp_setup_cpu_maps(void)
> >  	vdso_data->processorCount = num_present_cpus();
> >  #endif /* CONFIG_PPC64 */
> >  
> > -        /* Initialize CPU <=> thread mapping/
> > +	dn = of_find_node_by_type(NULL, "cpu");
> > +	if (dn) {
> > +		if (check_for_interleaved_big_core(dn)) {
> > +			has_interleaved_big_core = true;
> > +			pr_info("Detected interleaved big-cores\n");
> > +		}
> > +		of_node_put(dn);
> > +	}
> 
> This is a bit untidy, given how unlikely it is that you would have no
> CPUs :)

This can actually go into the earlier loop where we initialize the
smp_processor_ids(). I have fixed it in the next iteration.

> 
> You should be able to do the lookup of the property and the setting of
> has_interleaved_big_core all inside
> check_for_interleaved_big_core().

Yes, that's what I am doing in the next iteration.

> 
> cheers
>