[PATCH 1/2] tm27: Use generic cpu index accessor

[Sebastian Huber]

On 11/12/2020 16:09, Kinsey Moore wrote:

> -----Original Message-----
> From: Sebastian Huber<sebastian.huber at embedded-brains.de>  
> Sent: Thursday, December 10, 2020 23:48
> To: Kinsey Moore<kinsey.moore at oarcorp.com>;devel at rtems.org
> Subject: Re: [PATCH 1/2] tm27: Use generic cpu index accessor
>
>> On 10/12/2020 21:42, Kinsey Moore wrote:
>>> The arm_cp15 function for accessing the current CPU index is specific
>>> to ARMv7 while this header is used for ARMv8 as well. Instead, use a
>>> generic accessor that is part of the standard CPU API.
>> I am fine with this fix, however, this is basically now the same
>> approach as in my v2 patch:
>>
>> https://lists.rtems.org/pipermail/devel/2020-December/063693.html
>>
>> Since the tm27 test is configured to use only one CPU, the current
>> processor is always 0.
> Unfortunately, that's the only API currently available to get that information easily on both architectures. The base function that is wrapped (_CPU_SMP_Get_current_processor) could be provided outside the RTEMS_SMP ifdef block and that could be used instead since this is specific to ARM/AArch64. Would that be acceptable?

If _CPU_SMP_Get_current_processor() returns != 0 if 
CONFIGURE_MAXIMUM_PROCESSORS == 1, then this is undefined behaviour, 
since this value is used to index _Per_CPU_Information.

I think for now, the patch is fine.

What we have is an API mismatch between the logical processor index and 
the MPIDR register. Currently, a one-to-one mapping is assumed. For a 
proper support of clustered systems we may need a more complex mapping 
(for example a four core system consisting of a cluster (aff1: 0 or 1) 
of two cores each (aff0: 0 or 1)).

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