[PATCH v2] xen/arm: register clocks used by the hypervisor

[Dirk Behme]

Some clocks might be used by the Xen hypervisor and not by the Linux
kernel. If these are not registered by the Linux kernel, they might be
disabled by clk_disable_unused() as the kernel doesn't know that they
are used. The clock of the serial console handled by Xen is one
example for this. It might be disabled by clk_disable_unused() which
stops the whole serial output, even from Xen, then.

Up to now, the workaround for this has been to use the Linux kernel
command line parameter 'clk_ignore_unused'. See Xen bug

http://bugs.xenproject.org/xen/bug/45

too.

To fix this, we will add the "unused" clocks in Xen to the hypervisor
node. The Linux kernel has to register the clocks from the hypervisor
node, then.

Therefore, check if there is a "clocks" entry in the hypervisor node
and if so register the given clocks to the Linux kernel clock
framework and with this mark them as used. This prevents the clocks
from being disabled.

Signed-off-by: Dirk Behme <dirk.behme at de.bosch.com>
---
Changes in v2:
 - Rebase against git://git.kernel.org/pub/scm/linux/kernel/git/xen/tip.git for-linus-4.8
 - Add changes to Documentation/devicetree/bindings/arm/xen.txt

 Documentation/devicetree/bindings/arm/xen.txt | 11 +++++++
 arch/arm/xen/enlighten.c                      | 47 +++++++++++++++++++++++++++
 2 files changed, 58 insertions(+)

diff --git a/Documentation/devicetree/bindings/arm/xen.txt b/Documentation/devicetree/bindings/arm/xen.txt
index c9b9321..55dfd3b 100644
--- a/Documentation/devicetree/bindings/arm/xen.txt
+++ b/Documentation/devicetree/bindings/arm/xen.txt
@@ -17,6 +17,17 @@ the following properties:
   A GIC node is also required.
   This property is unnecessary when booting Dom0 using ACPI.
 
+Optional properties:
+
+- clocks: one or more clocks to be registered.
+  Xen hypervisor drivers might replace native drivers, resulting in
+  clocks not registered by these native drivers. To avoid that these
+  unregistered clocks are disabled, then, e.g. by clk_disable_unused(),
+  register them in the hypervisor node.
+  An example for this are the clocks of the serial driver. If the clocks
+  used by the serial hardware interface are not registered by the serial
+  driver the serial output might stop once clk_disable_unused() is called.
+
 To support UEFI on Xen ARM virtual platforms, Xen populates the FDT "uefi" node
 under /hypervisor with following parameters:
 
diff --git a/arch/arm/xen/enlighten.c b/arch/arm/xen/enlighten.c
index 47acb36..5c546d0 100644
--- a/arch/arm/xen/enlighten.c
+++ b/arch/arm/xen/enlighten.c
@@ -24,6 +24,7 @@
 #include <linux/of_fdt.h>
 #include <linux/of_irq.h>
 #include <linux/of_address.h>
+#include <linux/clk-provider.h>
 #include <linux/cpuidle.h>
 #include <linux/cpufreq.h>
 #include <linux/cpu.h>
@@ -444,6 +445,52 @@ static int __init xen_pm_init(void)
 }
 late_initcall(xen_pm_init);
 
+/*
+ * Check if we want to register some clocks, that they
+ * are not freed because unused by clk_disable_unused().
+ * E.g. the serial console clock.
+ */
+static int __init xen_arm_register_clks(void)
+{
+	struct clk *clk;
+	struct device_node *xen_node;
+	unsigned int i, count;
+	int ret = 0;
+
+	xen_node = of_find_compatible_node(NULL, NULL, "xen,xen");
+	if (!xen_node) {
+		pr_err("Xen support was detected before, but it has disappeared\n");
+		return -EINVAL;
+	}
+
+	count = of_clk_get_parent_count(xen_node);
+	if (!count)
+		goto out;
+
+	for (i = 0; i < count; i++) {
+		clk = of_clk_get(xen_node, i);
+		if (IS_ERR(clk)) {
+			pr_err("Xen failed to register clock %i. Error: %li\n",
+			       i, PTR_ERR(clk));
+			ret = PTR_ERR(clk);
+			goto out;
+		}
+
+		ret = clk_prepare_enable(clk);
+		if (ret < 0) {
+			pr_err("Xen failed to enable clock %i. Error: %i\n",
+			       i, ret);
+			goto out;
+		}
+	}
+
+	ret = 0;
+
+out:
+	of_node_put(xen_node);
+	return ret;
+}
+late_initcall(xen_arm_register_clks);
 
 /* empty stubs */
 void xen_arch_pre_suspend(void) { }
-- 
2.8.0