[Question] Is little endian supported on all the platforms?

C├ędric Le Goater clg at fr.ibm.com
Mon Aug 31 22:12:54 AEST 2015


On 08/31/2015 01:19 PM, Michael Ellerman wrote:
> On Mon, 2015-08-31 at 15:53 +0800, Boqun Feng wrote:
>> On Mon, Aug 31, 2015 at 04:52:38PM +1000, Benjamin Herrenschmidt wrote:
>>> On Mon, 2015-08-31 at 14:44 +0800, Boqun Feng wrote:
>>>> Hi all,
>>>>
>>>> I hit a strange build error on v4.2, when I try to build a LE kernel
>>>> with a slightly modification of the ppc64_defconfig. What I did is just
>>>> make ppc64_defconfig and make menuconfig to set CPU_LITTLE_ENDIAN=y, and
>>>> then build the kernel.
>>>>
>>>> I did a little research myself, and found out the error is because of
>>>> trying to build a LE kernel with CONFIG_PPC_PS3=y. So a simple way to
>>>> reproduce is:
>>>
>>> First, LE isn't supported on PS/3 :-) It's only supported on POWER8
>>> machines (and *some* P7 machines and only when running as KVM guest).
>>
>> Got it, thank you ;-)
>>
>> So should we modify Kconfigs of all the platform to reflect this?
> 
> Yes. Maybe something like this?
> 
> diff --git a/arch/powerpc/platforms/Kconfig.cputype b/arch/powerpc/platforms/Kconfig.cputype
> index c140e94..c359f72 100644
> --- a/arch/powerpc/platforms/Kconfig.cputype
> +++ b/arch/powerpc/platforms/Kconfig.cputype
> @@ -415,9 +415,13 @@ config VDSO32
>           big endian. That is because the only little endian configuration we
>           support is ppc64le which is 64-bit only.
>  
> +config CHOOSE_ENDIAN
> +       bool
> +
>  choice
>         prompt "Endianness selection"
>         default CPU_BIG_ENDIAN
> +       depends on CHOOSE_ENDIAN
>         help
>           This option selects whether a big endian or little endian kernel will
>           be built.
> diff --git a/arch/powerpc/platforms/powernv/Kconfig b/arch/powerpc/platforms/powernv/Kconfig
> index 604190c..32d5661 100644
> --- a/arch/powerpc/platforms/powernv/Kconfig
> +++ b/arch/powerpc/platforms/powernv/Kconfig
> @@ -18,6 +18,7 @@ config PPC_POWERNV
>         select CPU_FREQ_GOV_ONDEMAND
>         select CPU_FREQ_GOV_CONSERVATIVE
>         select PPC_DOORBELL
> +       select CHOOSE_ENDIAN
>         default y
>  
>  config OPAL_PRD
> diff --git a/arch/powerpc/platforms/pseries/Kconfig b/arch/powerpc/platforms/pseries/Kconfig
> index 54c87d5..182f485 100644
> --- a/arch/powerpc/platforms/pseries/Kconfig
> +++ b/arch/powerpc/platforms/pseries/Kconfig
> @@ -21,6 +21,7 @@ config PPC_PSERIES
>         select HOTPLUG_CPU if SMP
>         select ARCH_RANDOM
>         select PPC_DOORBELL
> +       select CHOOSE_ENDIAN
>         default y
>  
>  config PPC_SPLPAR


or like this, to be consistent with the other *_CPU configs in Kconfig.cputype :

--- a/arch/powerpc/platforms/ps3/Kconfig
+++ b/arch/powerpc/platforms/ps3/Kconfig
@@ -1,6 +1,6 @@
 config PPC_PS3
        bool "Sony PS3"
-       depends on PPC64 && PPC_BOOK3S
+       depends on PPC64 && PPC_BOOK3S && !CPU_LITTLE_ENDIAN
        select PPC_CELL
        select USB_OHCI_LITTLE_ENDIAN
        select USB_OHCI_BIG_ENDIAN_MMIO

> 
> 
>>> But the specific problem you are having looks like a differnet issue
>>> with the PS3 boot wrapper.
>>>
>>
>> But could the reason of this be that the building process of the PS3
>> boot wrapper assumes the kernel is BE?
> 
> It looks more like we're getting confused between 32-bit and 64-bit, from your
> log:
> 
>     ld: powerpc:common64 architecture of input file `arch/powerpc/boot/ps3-head.o' is incompatible with powerpc:common output
> 
> Which says it has a .o which is 64-bit but it's trying to produce 32-bit output.
> 
> That's probably related to the PPC64_BOOT_WRAPPER stuff.

yes. This is because CPU_LITTLE_ENDIAN selects PPC64_BOOT_WRAPPER.

Cheers,

C.



> 
> cheers
> 
> 
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