[OpenWrt-Devel] [RFC] Mips SIMD architecture (MSA)

[José Vázquez]

2015-04-13 12:28 GMT+02:00, Paul Burton <paul.burton at imgtec.com>:
> On Mon, Apr 13, 2015 at 11:59:20AM +0200, José Vázquez wrote:
>> 2015-04-13 11:15 GMT+02:00, Paul Burton <paul.burton at imgtec.com>:
>> > On Mon, Apr 13, 2015 at 10:54:05AM +0200, José Vázquez wrote:
>> >> As far i can understand MSA is a feature that is only present in the
>> >> MIPS Warrior cores and, for now, seems that could be only needed in
>> >> malta [1] target.
>> >> As you can see in arch/mips/Kconfig MIPS32R2 and MIPS64R2 select
>> >> CPU_SUPPORTS_MSA which does not apply to the majority of the mips
>> >> targets in OpenWRT.
>> >> Deleting "select CPU_SUPPORTS_MSA" in [2] and [3] and changing [4]
>> >> with "depends CPU_MIPS32_R2 || CPU_MIPS64_R2" will make MSA selectable
>> >> and should make the kernel smaller.
>> >
>> > Hello,
>> >
>> > I'm not sure I understand the problem here - CPU_HAS_MSA should already
>> > be configurable via Kconfig, so you can just disable it if you don't
>> > need it (as indicated in the Kconfig help text). The CPU_SUPPORTS_MSA
>> > entries are there just to enforce that you can only enable CPU_HAS_MSA
>> > on a system which might possibly include MSA (ie. MIPSr2+, well really
>> > MIPSr5+).
>> >
>> > In essence, the fact that your system selects CPU_SUPPORTS_MSA
>> > shouldn't
>> > increase the size of your kernel unless you also enable the
>> > configurable
>> > CPU_HAS_MSA. Am I missing something?
>> >
>> > Thanks,
>> >     Paul
>> >
>> >> [1]https://git.kernel.org/cgit/linux/kernel/git/stable/linux-stable.git/tree/arch/mips/Kconfig?id=refs/tags/v3.18.11#n311
>> >> [2]https://git.kernel.org/cgit/linux/kernel/git/stable/linux-stable.git/tree/arch/mips/Kconfig?id=refs/tags/v3.18.11#n1256
>> >> [3]https://git.kernel.org/cgit/linux/kernel/git/stable/linux-stable.git/tree/arch/mips/Kconfig?id=refs/tags/v3.18.11#n1290
>> >> [4]https://git.kernel.org/cgit/linux/kernel/git/stable/linux-stable.git/tree/arch/mips/Kconfig?id=refs/tags/v3.18.11#n2135
>> >>
>> >> Regards:
>> >>
>> >> Pepe
>> >>
>> MSA is not selectable if in Kconfig the target selects
>> SYS_HAS_CPU_MIPS32_R2 or SYS_HAS_CPU_MIPS64_R2, like Lantiq, PMC_MSP,
>> Ralink and some others because of [2] and [3].
>> I.e., in the case of EVA it is only selected if SYS_HAS_CPU_MIPS32_R3_5.
>>
>> Pepe
>
> ...but [2] & [3] both only select CPU_SUPPORTS_MSA, not CPU_HAS_MSA. You
> should still be able to disable CPU_HAS_MSA in your configuration.
> Nothing depends upon or selects CPU_HAS_MSA, so you should always be
> able to disable it regardless of the platform you build for.
>
> To take one of your examples I just checked out v3.18.11, started from
> the Lantiq xway_defconfig and ran menuconfig. MSA is disabled by default
> as expected, and can be enabled if (& only if) its dependency
> CONFIG_MIPS_O32_FP64_SUPPORT is enabled. Even when that is enabled MSA
> is still disabled by default.
>
> I still don't see what you're trying to do here - MSA can already always
> be disabled via Kconfig, and is automatically enforced disabled on
> systems where it doesn't make sense.
>
> Thanks,
>     Paul
>
You are right. Sorry and thanks for the clarification.

Best regards:

Pepe
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