[PATCH for-4.8 12/12] powerpc/mm/radix: Add a kernel command line to disable radix

[Balbir Singh]

On Wed, Jul 13, 2016 at 03:05:31PM +0530, Aneesh Kumar K.V wrote:
> This patch adds the kernel command line disable_radix which disable
> the radix MMU mode even if firmware indicates radix support via
> ibm,pa-features device tree node.
> 
> This helps in testing different MMU mode easily.
> 
> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar at linux.vnet.ibm.com>
> ---
>  Documentation/kernel-parameters.txt |  3 +++
>  arch/powerpc/kernel/prom.c          | 13 +++++++++++++
>  2 files changed, 16 insertions(+)
> 
> diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
> index 738bae4a5958..bba7ef30d74e 100644
> --- a/Documentation/kernel-parameters.txt
> +++ b/Documentation/kernel-parameters.txt
> @@ -929,6 +929,9 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
>  	disable=	[IPV6]
>  			See Documentation/networking/ipv6.txt.
>  
> +	disable_radix	[PPC]
> +			Disable RADIX MMU mode on POWER9
> +
>  	disable_cpu_apicid= [X86,APIC,SMP]
>  			Format: <int>
>  			The number of initial APIC ID for the
> diff --git a/arch/powerpc/kernel/prom.c b/arch/powerpc/kernel/prom.c
> index 946e34ffeae9..022540b9366d 100644
> --- a/arch/powerpc/kernel/prom.c
> +++ b/arch/powerpc/kernel/prom.c
> @@ -645,6 +645,14 @@ static void __init early_reserve_mem(void)
>  #endif
>  }
>  
> +static bool disable_radix;

__read_mostly?

> +static int __init parse_disable_radix(char *p)
> +{
> +	disable_radix = true;
> +	return 0;
> +}
> +early_param("disable_radix", parse_disable_radix);
> +
>  void __init early_init_devtree(void *params)
>  {
>  	phys_addr_t limit;
> @@ -739,6 +747,11 @@ void __init early_init_devtree(void *params)
>  	/* Scan and build the list of machine check recoverable ranges */
>  	of_scan_flat_dt(early_init_dt_scan_recoverable_ranges, NULL);
>  #endif
> +	/*
> +	 * now fixup radix MMU mode based on kernel command line
> +	 */
> +	if (disable_radix)
> +		cur_cpu_spec->mmu_features &= ~MMU_FTR_RADIX;

Shouldn't this happen before prom_init() and exchanging capabilities?