Re: [PATCH 2/2] arm64: topology: Use PPTT to determine if PE is a thread

[John Garry <john.garry@huawei.com>]

On 23/05/2019 23:40, Jeremy Linton wrote:
> ACPI 6.3 adds a thread flag to represent if a CPU/PE is
> actually a thread. Given that the MPIDR_MT bit may not
> represent this information consistently on homogeneous machines
> we should prefer the PPTT flag if its available.
>

Hi Jeremy,

I was just wondering if we should look to get this support backported 
(when merged)?

I worry about the case of a system with the CPU having MT bit in the 
MPIDR (while not actually threaded), i.e. the system for which these 
PPTT flags were added (as I understand).

> Signed-off-by: Jeremy Linton <jeremy.linton@arm.com>
> ---
>  arch/arm64/kernel/topology.c | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
>
> diff --git a/arch/arm64/kernel/topology.c b/arch/arm64/kernel/topology.c
> index 0825c4a856e3..cbbedb53cf06 100644
> --- a/arch/arm64/kernel/topology.c
> +++ b/arch/arm64/kernel/topology.c
> @@ -346,11 +346,9 @@ void remove_cpu_topology(unsigned int cpu)
>   */
>  static int __init parse_acpi_topology(void)
>  {
> -	bool is_threaded;
> +	int is_threaded;
>  	int cpu, topology_id;
>
> -	is_threaded = read_cpuid_mpidr() & MPIDR_MT_BITMASK;
> -
>  	for_each_possible_cpu(cpu) {
>  		int i, cache_id;
>
> @@ -358,6 +356,10 @@ static int __init parse_acpi_topology(void)
>  		if (topology_id < 0)
>  			return topology_id;
>
> +		is_threaded = acpi_pptt_cpu_is_thread(cpu);
> +		if (is_threaded < 0)
> +			is_threaded = read_cpuid_mpidr() & MPIDR_MT_BITMASK;
> +
>  		if (is_threaded) {
>  			cpu_topology[cpu].thread_id = topology_id;

For described above scenario, this seems wrong.

>  			topology_id = find_acpi_cpu_topology(cpu, 1);
>

BTW, we did test an old kernel with 6.3 PPTT bios for this on D06 (some 
versions have MT bit set), and it looked ok. But I am still a bit skeptical.

Thanks,
John