metag: init common clk and use "core" clk

If the common clock framework is enabled, call of_clk_init(NULL) in
time_init() to register device tree clocks with the clock framework.

After this time_init() calls a new function init_metag_clocks(), which
looks for a clock named "core" in the node compatible with "img,meta"
(usually the root node). If found the get_core_freq machine callback is
overridden to obtain the core clock frequency using that clock.

Signed-off-by: James Hogan <[e-mail]>
Cc: Mike Turquette <[e-mail]>
Cc: Grant Likely <[e-mail]>
Cc: Rob Herring <[e-mail]>
Cc: [e-mail]

Documentation/devicetree/bindings/metag/meta.txt

···
+* Meta Processor Binding
+
+This binding specifies what properties must be available in the device tree
+representation of a Meta Processor Core, which is the root node in the tree.
+
+Required properties:
+
+    - compatible: Specifies the compatibility list for the Meta processor.
+      The type shall be <string> and the value shall include "img,meta".
+
+Optional properties:
+
+    - clocks: Clock consumer specifiers as described in
+      Documentation/devicetree/bindings/clock/clock-bindings.txt
+
+    - clock-names: Clock consumer names as described in
+      Documentation/devicetree/bindings/clock/clock-bindings.txt.
+
+Clocks are identified by name. Valid clocks are:
+
+    - "core": The Meta core clock from which the Meta timers are derived.
+
+* Examples
+
+/ {
+	compatible = "toumaz,tz1090", "img,meta";
+
+	clocks = <&meta_core_clk>;
+	clock-names = "core";
+};

arch/metag/include/asm/clock.h

···
  *			core frequency will be determined like this:
  *			Meta 1: based on loops_per_jiffy.
  *			Meta 2: (EXPAND_TIMER_DIV + 1) MHz.
+ *			If a "core" clock is provided by the device tree, it
+ *			will override this function.
  */
 struct meta_clock_desc {
 	unsigned long		(*get_core_freq)(void);
 };
 
 extern struct meta_clock_desc _meta_clock;
+
+/*
+ * Perform platform clock initialisation, reading clocks from device tree etc.
+ * Only accessible during boot.
+ */
+void init_metag_clocks(void);
 
 /*
  * Set up the default clock, ensuring all callbacks are valid - only accessible

arch/metag/kernel/clock.c

···
  * published by the Free Software Foundation.
  */
 
+#include <linux/clk.h>
 #include <linux/delay.h>
 #include <linux/io.h>
+#include <linux/of.h>
 
 #include <asm/param.h>
 #include <asm/clock.h>
···
 #endif
 }
 
+static struct clk *clk_core;
+
+/* Clk based get_core_freq callback. */
+static unsigned long get_core_freq_clk(void)
+{
+	return clk_get_rate(clk_core);
+}
+
 /**
- * setup_meta_clocks() - Set up the Meta clock.
+ * init_metag_core_clock() - Set up core clock from devicetree.
+ *
+ * Checks to see if a "core" clock is provided in the device tree, and overrides
+ * the get_core_freq callback to use it.
+ */
+static void __init init_metag_core_clock(void)
+{
+	/*
+	 * See if a core clock is provided by the devicetree (and
+	 * registered by the init callback above).
+	 */
+	struct device_node *node;
+	node = of_find_compatible_node(NULL, NULL, "img,meta");
+	if (!node) {
+		pr_warn("%s: no compatible img,meta DT node found\n",
+			__func__);
+		return;
+	}
+
+	clk_core = of_clk_get_by_name(node, "core");
+	if (IS_ERR(clk_core)) {
+		pr_warn("%s: no core clock found in DT\n",
+			__func__);
+		return;
+	}
+
+	/*
+	 * Override the core frequency callback to use
+	 * this clk.
+	 */
+	_meta_clock.get_core_freq = get_core_freq_clk;
+}
+
+/**
+ * init_metag_clocks() - Set up clocks from devicetree.
+ *
+ * Set up important clocks from device tree. In particular any needed for clock
+ * sources.
+ */
+void __init init_metag_clocks(void)
+{
+	init_metag_core_clock();
+}
+
+/**
+ * setup_meta_clocks() - Early set up of the Meta clock.
  * @desc:	Clock descriptor usually provided by machine description
  *
  * Ensures all callbacks are valid.

arch/metag/kernel/time.c

···
  *
  */
 
-#include <linux/init.h>
-
 #include <clocksource/metag_generic.h>
+#include <linux/clk-provider.h>
+#include <linux/init.h>
+#include <asm/clock.h>
 
 void __init time_init(void)
 {
+#ifdef CONFIG_COMMON_CLK
+	/* Init clocks from device tree */
+	of_clk_init(NULL);
+#endif
+
+	/* Init meta clocks, particularly the core clock */
+	init_metag_clocks();
+
+	/* Set up the timer clock sources */
 	metag_generic_timer_init();
 }