Re: [PATCH v3 6/7] arm64: topology: Enable ACPI/PPTT based CPU topology.

[Sudeep Holla <sudeep.holla@arm.com>]

On 20/10/17 17:14, Jeremy Linton wrote:
> Hi,
> 
> On 10/20/2017 04:14 AM, Lorenzo Pieralisi wrote:
>> On Thu, Oct 19, 2017 at 11:13:27AM -0500, Jeremy Linton wrote:
>>> On 10/19/2017 10:56 AM, Lorenzo Pieralisi wrote:
>>>> On Thu, Oct 12, 2017 at 02:48:55PM -0500, Jeremy Linton wrote:
>>>>> Propagate the topology information from the PPTT tree to the
>>>>> cpu_topology array. We can get the thread id, core_id and
>>>>> cluster_id by assuming certain levels of the PPTT tree correspond
>>>>> to those concepts. The package_id is flagged in the tree and can be
>>>>> found by passing an arbitrary large level to setup_acpi_cpu_topology()
>>>>> which terminates its search when it finds an ACPI node flagged
>>>>> as the physical package. If the tree doesn't contain enough
>>>>> levels to represent all of thread/core/cod/package then the package
>>>>> id will be used for the missing levels.
>>>>>
>>>>> Since server/ACPI machines are more likely to be multisocket and NUMA,
>>>>
>>>> I think this stuff is vague enough already so to start with I would
>>>> drop
>>>> patch 4 and 5 and stop assuming what machines are more likely to ship
>>>> with ACPI than DT.
>>>>
>>>> I am just saying, for the umpteenth time, that these levels have no
>>>> architectural meaning _whatsoever_, level is a hierarchy concept
>>>> with no architectural meaning attached.
>>>
>>> ?
>>>
>>> Did anyone say anything about that? No, I think the only thing being
>>> guaranteed here is that the kernel's physical_id maps to an ACPI
>>> defined socket. Which seems to be the mindset of pretty much the
>>> entire !arm64 community meaning they are optimizing their software
>>> and the kernel with that concept in mind.
>>>
>>> Are you denying the existence of non-uniformity between threads
>>> running on different physical sockets?
>>
>> No, I have not explained my POV clearly, apologies.
>>
>> AFAIK, the kernel currently deals with 2 (3 - if SMT) topology layers.
>>
>> 1) thread
>> 2) core
>> 3) package
>>
>> What I wanted to say is, that, to simplify this series, you do not need
>> to introduce the COD topology level, since it is just another arbitrary
>> topology level (ie there is no way you can pinpoint which level
>> corresponds to COD with PPTT - or DT for the sake of this discussion)
>> that would not be used in the kernel (apart from big.LITTLE cpufreq
>> driver and PSCI checker whose usage of topology_physical_package_id() is
>> questionable anyway).

Just thinking out loud here.

1. psci_checker.c : it's just used to get groups of cpu's to achieve
   deeper idle states. It should be easy to get rid of that.
2. big.LITTLE cpufreq : 2 users, scpi I should be able to do what I did
   for SCMI and for spc I am thinking if we can hard code it's just used
   on TC2

-- 
Regards,
Sudeep