Re: [Buildroot] [PATCH 3/9] arch/powerpc: Enable powerpc64le only on CPUs that support it

["Cédric Le Goater" <clg@kaod.org>]

On 9/18/22 12:23, Arnout Vandecappelle wrote:
> 
> 
> On 29/07/2022 02:08, Joel Stanley wrote:
>> Invalid configurations lead to build failures, such as trying to enable
>> ppc64le for the ppc970:
>>
>>    http://autobuild.buildroot.net/results/fda886768fce25ccd9b52b635ff5b13da7ba2d0c/
>>
>> In order to run a ppc64le userspace a kernel that runs in this mode is
>> required. The only CPU supported in buildroot that can boot a ppc64le
>> kernel is Power8, so mark all of the other 64-bit capable CPUs as not
>> supporting ppc64le.
>>
>> This drops the comment about libc, which is true but doesn't tell the
>> whole story.
>>
>> Signed-off-by: Joel Stanley <joel@jms.id.au>
> 
>   Applied to master with some changes, see below.
> 
>> ---
>>   arch/Config.in.powerpc | 11 +++++++----
>>   1 file changed, 7 insertions(+), 4 deletions(-)
>>
>> diff --git a/arch/Config.in.powerpc b/arch/Config.in.powerpc
>> index 8d392bfde814..7c6ae84348e9 100644
>> --- a/arch/Config.in.powerpc
>> +++ b/arch/Config.in.powerpc
>> @@ -71,11 +71,9 @@ config BR2_powerpc_604e
>>       depends on !BR2_ARCH_IS_64
>>   config BR2_powerpc_620
>>       bool "620"
>> -    # No C library supports this variant on ppc64le
>>       depends on !BR2_powerpc64le
>>   config BR2_powerpc_630
>>       bool "630"
>> -    # No C library supports this variant on ppc64le
>>       depends on !BR2_powerpc64le
>>   config BR2_powerpc_740
>>       bool "740"
>> @@ -102,6 +100,7 @@ config BR2_powerpc_860
>>       depends on !BR2_ARCH_IS_64
>>   config BR2_powerpc_970
>>       bool "970"
>> +    depends on !BR2_powerpc64le
> 
>   As suggested by Thomas, I changed this to positive logic. I'll also push a follow-up patch that changes the rest of the file to positive logic.
> 
>   Could you (or any other powerpc "expert") review the conditions to be sure the architecture selection options are correct now?
> 
>>       select BR2_POWERPC_CPU_HAS_ALTIVEC
>>   config BR2_powerpc_8540
>>       bool "8540 / e500v1"
>> @@ -129,21 +128,25 @@ config BR2_powerpc_e6500
>>       select BR2_POWERPC_CPU_HAS_ALTIVEC
>>   config BR2_powerpc_power4
>>       bool "power4"
>> -    # No C library supports this variant on ppc64le
>>       depends on !BR2_powerpc64le
>>   config BR2_powerpc_power5
>>       bool "power5"
>> -    # No C library supports this variant on ppc64le
>>       depends on !BR2_powerpc64le
>> +    depends on BR2_ARCH_IS_64
> 
>   AFAIU, all POWER CPUs are able to run in 32-bit mode 

yes. The question is more : can we generate correctly 32-bit software
for all these CPUs ? firmware, kernel and userspace. I doubt we have
correct support for older CPUs.

> and I believe it's even possible to run a 64-bit kernel with 32-bit userspace like on ARM. 
> Is this not true?

It is. This is checked on older CPUs, like G5, under QEMU but 32bit is
bit-rotting in many places.

> 
>   If POWER5+ really can't run in 32-bit mode (or this is not supported by the kernel), 

I think there is an issue with the use of the rfi instruction in 32-bit
mode which was removed from the ISA of server CPUs. You can not generate
a 32bit kernel for such CPUs AFAIR

> are you sure that this *is* possible on POWER4?

POWER4 support was removed from Linux ~4 years ago.
  
>   In either case, if the current conditions are not correct, please send 
> a follow-up patch to fix them even more.

It is safer to build all 64-bit (kernel + userspace) for server CPUs.
  
>>   config BR2_powerpc_power6
>>       bool "power6"
>> +    depends on !BR2_powerpc64le
>> +    depends on BR2_ARCH_IS_64
> 
>   So here I wrote the condition as
> 
>      depends on BR2_powerpc || BR2_powerpc64
> 
> because I do believe it supports 32-bit.

These definitions can be problematic :

   config BR2_powerpc_powerx
	  bool "powerx"
	  depends on BR2_powerpc || BR2_powerpc64

because they imply that the kernel could be built in 32bit. See above.

Thanks,

C.

> 
>   Regards,
>   Arnout
> 
>>       select BR2_POWERPC_CPU_HAS_ALTIVEC
>>   config BR2_powerpc_power7
>>       bool "power7"
>> +    depends on !BR2_powerpc64le
>> +    depends on BR2_ARCH_IS_64
>>       select BR2_POWERPC_CPU_HAS_ALTIVEC
>>       select BR2_POWERPC_CPU_HAS_VSX
>>   config BR2_powerpc_power8
>>       bool "power8"
>> +    depends on BR2_ARCH_IS_64
>>       select BR2_POWERPC_CPU_HAS_ALTIVEC
>>       select BR2_POWERPC_CPU_HAS_VSX
>>   endchoice

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