linux: Set MMU page size for ARC processors

ARC processors have configurable size of MMU page. This configuration
happens during ASIC design and couldn't be changed in final silicone
not to mention runtime changes.

Given PAGE_SIZE macro is used a lot throughout the Linux kernel sources
we just hardcode a required value during the kernel configuration.

We used to support different MMU page sizes for ARC in Buildroot for
quite some time now but so far we only tweaked uClibc on the matter.
That left us with the kernel configured with whatever was in used defconfig.

In most of real cases that's OK because typically we're building firmware
for a particular ASIC which is supposed to have a unique kernel defconfig.
But if we're dealing with FPGA-based boards or even simlators like
Synopsys DesignWare nSIM or QEMU it's possible to have dfferent MMU page
size configured in that target mostly for the sake of testing.

And so we're trying to solve 2 problems here:
 1. Make sure both user-space (via libc settings) and the Linux kernel
    are "on the same page", i.e. expect to use the same MMU page size.
 2. Simplify process of testing different page sizes.
    As now we first need to set page size in Buildroot and then in the
    kernel via "make linux-configure" or via Kconfig fragment.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
---
 linux/linux.mk | 12 ++++++++++++
 1 file changed, 12 insertions(+)

Alexey, All,

On 2019-12-17 16:15 +0300, Alexey Brodkin spake thusly:
> ARC processors have configurable size of MMU page. This configuration
> happens during ASIC design and couldn't be changed in final silicone
> not to mention runtime changes.
> 
> Given PAGE_SIZE macro is used a lot throughout the Linux kernel sources
> we just hardcode a required value during the kernel configuration.
> 
> We used to support different MMU page sizes for ARC in Buildroot for
> quite some time now but so far we only tweaked uClibc on the matter.
> That left us with the kernel configured with whatever was in used defconfig.
> 
> In most of real cases that's OK because typically we're building firmware
> for a particular ASIC which is supposed to have a unique kernel defconfig.
> But if we're dealing with FPGA-based boards or even simlators like
> Synopsys DesignWare nSIM or QEMU it's possible to have dfferent MMU page
> size configured in that target mostly for the sake of testing.
> 
> And so we're trying to solve 2 problems here:
>  1. Make sure both user-space (via libc settings) and the Linux kernel
>     are "on the same page", i.e. expect to use the same MMU page size.
>  2. Simplify process of testing different page sizes.
>     As now we first need to set page size in Buildroot and then in the
>     kernel via "make linux-configure" or via Kconfig fragment.
> 
> Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
> ---
>  linux/linux.mk | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
> 
> diff --git a/linux/linux.mk b/linux/linux.mk
> index 762f8868b1..3b83900c74 100644
> --- a/linux/linux.mk
> +++ b/linux/linux.mk
> @@ -319,6 +319,18 @@ define LINUX_KCONFIG_FIXUP_CMDS
>  		$(call KCONFIG_ENABLE_OPT,CONFIG_AEABI,$(@D)/.config))
>  	$(if $(BR2_powerpc)$(BR2_powerpc64)$(BR2_powerpc64le),
>  		$(call KCONFIG_ENABLE_OPT,CONFIG_PPC_DISABLE_WERROR,$(@D)/.config))
> +	$(and $(BR2_arc),$(BR2_ARC_PAGE_SIZE_4K),

Although I like this trick of using $(and ...) to test multiple
conditions, this is here not needed after all.

Indeed, if BR2_ARC_PAGE_SIZE_4K is set, then BR2_arc is also set,
because BR2_ARC_PAGE_SIZE_4K depends on BR2_arc.

So, the conditions can be simplified as:

    $(if $(BR2_ARC_PAGE_SIZE_4K),
        [...])

Ditto for 8KiB and 16KiB, of course.

I've fixed that and applied to master. Thanks.

Regards,
Yann E. MORIN.

> +		$(call KCONFIG_ENABLE_OPT,CONFIG_ARC_PAGE_SIZE_4K,$(@D)/.config)
> +		$(call KCONFIG_DISABLE_OPT,CONFIG_ARC_PAGE_SIZE_8K,$(@D)/.config)
> +		$(call KCONFIG_DISABLE_OPT,CONFIG_ARC_PAGE_SIZE_16K,$(@D)/.config))
> +	$(and $(BR2_arc),$(BR2_ARC_PAGE_SIZE_8K),
> +		$(call KCONFIG_DISABLE_OPT,CONFIG_ARC_PAGE_SIZE_4K,$(@D)/.config)
> +		$(call KCONFIG_ENABLE_OPT,CONFIG_ARC_PAGE_SIZE_8K,$(@D)/.config)
> +		$(call KCONFIG_DISABLE_OPT,CONFIG_ARC_PAGE_SIZE_16K,$(@D)/.config))
> +	$(and $(BR2_arc),$(BR2_ARC_PAGE_SIZE_16K),
> +		$(call KCONFIG_DISABLE_OPT,CONFIG_ARC_PAGE_SIZE_4K,$(@D)/.config)
> +		$(call KCONFIG_DISABLE_OPT,CONFIG_ARC_PAGE_SIZE_8K,$(@D)/.config)
> +		$(call KCONFIG_ENABLE_OPT,CONFIG_ARC_PAGE_SIZE_16K,$(@D)/.config))
>  	$(if $(BR2_TARGET_ROOTFS_CPIO),
>  		$(call KCONFIG_ENABLE_OPT,CONFIG_BLK_DEV_INITRD,$(@D)/.config))
>  	# As the kernel gets compiled before root filesystems are
> -- 
> 2.16.2
> 
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