[PATCH 1/5] ARM: bL_switcher: Don't enable bL switcher on systems without CCI

[Nicolas Pitre]

On Fri, 11 Apr 2014, Abhilash Kesavan wrote:

> From: Andrew Bresticker <abrestic at chromium.org>
> 
> Do not enable the big.LITTLE switcher on systems that do not have an
> ARM CCI (cache-coherent interconnect) present.  Since the CCI is used
> to maintain cache coherency between multiple clusters and peripherals,
> it is unlikely that a system without CCI would support big.LITTLE.
> 
> Signed-off-by: Andrew Bresticker <abrestic at chromium.org>
> Signed-off-by: Abhilash Kesavan <a.kesavan at samsung.com>

The b.L switcher depends on MCPM, and it also expects only 2 clusters 
which is evaluated at run time or it bails out.

There might be in theory other ways than the CCI to enforce coherency 
between clusters.  And that should all be encapsulated by the MCPM 
layer.  The switcher module should not be concerned at all by the 
underlying hardware mechanism.

So if the goal is to determine at run time whether or not the switcher 
should be activated in a multi-config kernel, then the criteria should 
be whether or not MCPM is initialized, and not if there is a CCI.


> ---
>  arch/arm/common/bL_switcher.c |   12 ++++++++++++
>  1 file changed, 12 insertions(+)
> 
> diff --git a/arch/arm/common/bL_switcher.c b/arch/arm/common/bL_switcher.c
> index 5774b6e..c4fec1f 100644
> --- a/arch/arm/common/bL_switcher.c
> +++ b/arch/arm/common/bL_switcher.c
> @@ -33,6 +33,7 @@
>  #include <linux/sysfs.h>
>  #include <linux/irqchip/arm-gic.h>
>  #include <linux/moduleparam.h>
> +#include <linux/of.h>
>  
>  #include <asm/smp_plat.h>
>  #include <asm/cputype.h>
> @@ -796,6 +797,17 @@ core_param(no_bL_switcher, no_bL_switcher, bool, 0644);
>  static int __init bL_switcher_init(void)
>  {
>  	int ret;
> +	struct device_node *dn;
> +
> +	/*
> +	 * We don't want to set up the bL switcher if the machine doesn't
> +	 * support bL, so use the presence of a CCI to indicate whether or
> +	 * not bL is supported on this device.
> +	 */
> +	dn = of_find_compatible_node(NULL, NULL, "arm,cci-400");
> +	if (!dn)
> +		return 0;
> +	of_node_put(dn);
>  
>  	if (MAX_NR_CLUSTERS != 2) {
>  		pr_err("%s: only dual cluster systems are supported\n", __func__);
> -- 
> 1.7.9.5
>