[RFC] Mips SIMD architecture (MSA)

[José Vázquez]

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.

[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

[Paul Burton]

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
>

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[José Vázquez]

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

[Paul Burton]

...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

[José Vázquez]

You are right. Sorry and thanks for the clarification.

Best regards:

Pepe