clk: Allow clocks to be marked as CRITICAL

Critical clocks are those which must not be gated, else undefined
or catastrophic failure would occur.  Here we have chosen to
ensure the prepare/enable counts are correctly incremented, so as
not to confuse users with enabled clocks with no visible users.

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Michael Turquette <mturquette@baylibre.com>
Link: lkml.kernel.org/r/1455225554-13267-2-git-send-email-mturquette@baylibre.com

drivers/clk/clk.c

@@ -2397,6 +2397,11 @@ static int __clk_core_init(struct clk_core *core)
 	if (core->ops->init)
 		core->ops->init(core->hw);
 
+	if (core->flags & CLK_IS_CRITICAL) {
+		clk_core_prepare(core);
+		clk_core_enable(core);
+	}
+
 	kref_init(&core->ref);
 out:
 	clk_prepare_unlock();

include/linux/clk-provider.h

@@ -32,6 +32,7 @@
 #define CLK_GET_ACCURACY_NOCACHE BIT(8) /* do not use the cached clk accuracy */
 #define CLK_RECALC_NEW_RATES	BIT(9) /* recalc rates after notifications */
 #define CLK_SET_RATE_UNGATE	BIT(10) /* clock needs to run to set rate */
+#define CLK_IS_CRITICAL		BIT(11) /* do not gate, ever */
 
 struct clk;
 struct clk_hw;