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

[Nicolas Pitre]

On Thu, 17 Apr 2014, Abhilash Kesavan wrote:

> Hi Nicolas,
> 
> On Fri, Apr 11, 2014 at 11:44 PM, Nicolas Pitre
> <nicolas.pitre at linaro.org> wrote:
> > 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.
> 
> Yes, we have a multi-SoC enabled kernel (with MCPM and BL_SWITCHER
> configs enabled); one SoC has a single cluster while the other is a
> dual cluster. We wanted a run-time check to prevent bL_switcher from
> running on the single cluster one and zeroed in on CCI. But, I get
> your point as to why CCI should not be used as a distinguishing factor
> for switcher initialization.
> 
> For now, I can use the no_bL_switcher parameter to disable it on
> certain platforms. However, can you elaborate on the MCPM
> initialization check you suggested ?

Here's what I mean:

----- >8

From: Nicolas Pitre <nicolas.pitre at linaro.org>
Date: Wed, 16 Apr 2014 15:43:59 -0400
Subject: [PATCH] ARM: bL_switcher: fix validation check before its activation

The switcher should not depend on MAX_CLUSTER which is a build time
limit to  determine ifit should be activated or not. In a multiplatform
kernel binary it is possible to have dual-cluster and quad-cluster
platforms configured in. In that case MAX_CLUSTER should be 4 and that
shouldn't prevent the switcher from working if the kernel is booted on a
b.L dual-cluster system.

In bL_switcher_halve_cpus() we already have a runtime validation check
to make sure we're dealing with only two clusters, so booting on a quad
cluster system will be caught and switcher activation aborted.

However, the b.L switcher must ensure the MCPM layer is initialized on
the booted hardware before doing anything.  The mcpm_is_available()
function is added to that effect.

Signed-off-by: Nicolas Pitre <nico at linaro.org>

diff --git a/arch/arm/common/bL_switcher.c b/arch/arm/common/bL_switcher.c
index 5774b6ea7a..f01c0ee0c8 100644
--- a/arch/arm/common/bL_switcher.c
+++ b/arch/arm/common/bL_switcher.c
@@ -797,10 +797,8 @@ static int __init bL_switcher_init(void)
 {
 	int ret;
 
-	if (MAX_NR_CLUSTERS != 2) {
-		pr_err("%s: only dual cluster systems are supported\n", __func__);
-		return -EINVAL;
-	}
+	if (!mcpm_is_available())
+		return -ENODEV;
 
 	cpu_notifier(bL_switcher_hotplug_callback, 0);
 
diff --git a/arch/arm/common/mcpm_entry.c b/arch/arm/common/mcpm_entry.c
index 1e361abc29..86fd60fefb 100644
--- a/arch/arm/common/mcpm_entry.c
+++ b/arch/arm/common/mcpm_entry.c
@@ -48,6 +48,11 @@ int __init mcpm_platform_register(const struct mcpm_platform_ops *ops)
 	return 0;
 }
 
+bool mcpm_is_available(void)
+{
+	return (platform_ops) ? true : false;
+}
+
 int mcpm_cpu_power_up(unsigned int cpu, unsigned int cluster)
 {
 	if (!platform_ops)
diff --git a/arch/arm/include/asm/mcpm.h b/arch/arm/include/asm/mcpm.h
index 608516ebab..a5ff410dcd 100644
--- a/arch/arm/include/asm/mcpm.h
+++ b/arch/arm/include/asm/mcpm.h
@@ -54,6 +54,13 @@ void mcpm_set_early_poke(unsigned cpu, unsigned cluster,
  */
 
 /**
+ * mcpm_is_available - returns whether MCPM is initialized and available
+ *
+ * This returns true or false accordingly.
+ */
+bool mcpm_is_available(void);
+
+/**
  * mcpm_cpu_power_up - make given CPU in given cluster runable
  *
  * @cpu: CPU number within given cluster