clk: mediatek: mux: add clk notifier functions

With device frequency scaling, the mux clock that (indirectly) feeds the
device selects between a dedicated PLL, and some other stable clocks.

When a clk rate change is requested, the (normally) upstream PLL is
reconfigured. It's possible for the clock output of the PLL to become
unstable during this process.

To avoid causing the device to glitch, the mux should temporarily be
switched over to another "stable" clock during the PLL rate change.
This is done with clk notifiers.

This patch adds common functions for notifiers to temporarily and
transparently reparent mux clocks.

This was loosely based on commit 8adfb08 ("clk: sunxi-ng: mux: Add
clk notifier functions").

Signed-off-by: Chen-Yu Tsai <wenst@chromium.org>
[Angelo: Changed mtk_mux_nb to hold a pointer to clk_ops instead of mtk_mux]
Co-developed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Reviewed-by: Miles Chen <miles.chen@mediatek.com>
Link: https://lore.kernel.org/r/20220927101128.44758-4-angelogioacchino.delregno@collabora.com
Signed-off-by: Chen-Yu Tsai <wenst@chromium.org>

Author: wens

drivers/clk/mediatek/clk-mux.c

@@ -4,6 +4,7 @@
  * Author: Owen Chen <owen.chen@mediatek.com>
  */
 
+#include <linux/clk.h>
 #include <linux/clk-provider.h>
 #include <linux/compiler_types.h>
 #include <linux/container_of.h>
@@ -259,4 +260,41 @@ void mtk_clk_unregister_muxes(const struct mtk_mux *muxes, int num,
 }
 EXPORT_SYMBOL_GPL(mtk_clk_unregister_muxes);
 
+/*
+ * This clock notifier is called when the frequency of the parent
+ * PLL clock is to be changed. The idea is to switch the parent to a
+ * stable clock, such as the main oscillator, while the PLL frequency
+ * stabilizes.
+ */
+static int mtk_clk_mux_notifier_cb(struct notifier_block *nb,
+				   unsigned long event, void *_data)
+{
+	struct clk_notifier_data *data = _data;
+	struct clk_hw *hw = __clk_get_hw(data->clk);
+	struct mtk_mux_nb *mux_nb = to_mtk_mux_nb(nb);
+	int ret = 0;
+
+	switch (event) {
+	case PRE_RATE_CHANGE:
+		mux_nb->original_index = mux_nb->ops->get_parent(hw);
+		ret = mux_nb->ops->set_parent(hw, mux_nb->bypass_index);
+		break;
+	case POST_RATE_CHANGE:
+	case ABORT_RATE_CHANGE:
+		ret = mux_nb->ops->set_parent(hw, mux_nb->original_index);
+		break;
+	}
+
+	return notifier_from_errno(ret);
+}
+
+int devm_mtk_clk_mux_notifier_register(struct device *dev, struct clk *clk,
+				       struct mtk_mux_nb *mux_nb)
+{
+	mux_nb->nb.notifier_call = mtk_clk_mux_notifier_cb;
+
+	return devm_clk_notifier_register(dev, clk, &mux_nb->nb);
+}
+EXPORT_SYMBOL_GPL(devm_mtk_clk_mux_notifier_register);
+
 MODULE_LICENSE("GPL");

drivers/clk/mediatek/clk-mux.h

@@ -7,12 +7,14 @@
 #ifndef __DRV_CLK_MTK_MUX_H
 #define __DRV_CLK_MTK_MUX_H
 
+#include <linux/notifier.h>
 #include <linux/spinlock.h>
 #include <linux/types.h>
 
 struct clk;
 struct clk_hw_onecell_data;
 struct clk_ops;
+struct device;
 struct device_node;
 
 struct mtk_mux {
@@ -89,4 +91,17 @@ int mtk_clk_register_muxes(const struct mtk_mux *muxes,
 void mtk_clk_unregister_muxes(const struct mtk_mux *muxes, int num,
 			      struct clk_hw_onecell_data *clk_data);
 
+struct mtk_mux_nb {
+	struct notifier_block	nb;
+	const struct clk_ops	*ops;
+
+	u8	bypass_index;	/* Which parent to temporarily use */
+	u8	original_index;	/* Set by notifier callback */
+};
+
+#define to_mtk_mux_nb(_nb)	container_of(_nb, struct mtk_mux_nb, nb)
+
+int devm_mtk_clk_mux_notifier_register(struct device *dev, struct clk *clk,
+				       struct mtk_mux_nb *mux_nb);
+
 #endif /* __DRV_CLK_MTK_MUX_H */