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

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Hi,

Thanks for testing and looking at this.

On 6/6/19 3:49 AM, John Garry wrote:
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 imagine that will happen..


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).

I have tested this patch on DAWN which happens to have the MT bit set, but isn't threaded, and it appears to work.


Signed-off-by: Jeremy Linton <jeremy.linton@xxxxxxx>
---
 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.

I'm not sure I understand the concern.

This is going to ignore the MPIDR_MT bit on any machine with a PPTT revision > 1. Are you worried about the topology_id assignment?



             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




Thanks,



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