clk: keystone: Add support AM62 DSS clock divider

On TI's AM62 Family of SoCs, the pixel frequency of the DSS Video Port 0
comes from a by-7 clock divider. This is done to support the clock for
OLDI transmitters that need serial clock 7 times the pixel frequency.

A clock set request on this clock, is forwarded to its parent clock by
default, using the SET_RATE_PARENT flag. The PLL (in this case the
parent) generates serial clock for OLDI, which is then also fed to DSS
using this clock divider.

Signed-off-by: Aradhya Bhatia <a-bhatia1@ti.com>

drivers/clk/keystone/Kconfig

@@ -34,3 +34,12 @@ config TI_SYSCON_CLK
 	help
 	  This adds clock driver support for syscon based gate
 	  clocks on TI's K2 and K3 SoCs.
+
+config TI_AM62_DSS_CLK
+	tristate "Clock Divider for DSS VP0 of AM62 Family of SoCs"
+	depends on ARCH_KEYSTONE || ARCH_K3 || COMPILE_TEST
+	default TI_SCI_CLK
+	help
+	  This adds clock divider driver support for Video Port 0 of Display
+	  SubSystems (DSS) under AM62 Family of SoCs. This clock divider
+	  forwards a seventh (1/7) of the incoming clock.

drivers/clk/keystone/Makefile

@@ -2,3 +2,4 @@
 obj-$(CONFIG_COMMON_CLK_KEYSTONE)	+= pll.o gate.o
 obj-$(CONFIG_TI_SCI_CLK)		+= sci-clk.o
 obj-$(CONFIG_TI_SYSCON_CLK)		+= syscon-clk.o
+obj-$(CONFIG_TI_AM62_DSS_CLK)		+= clk-am62-dss.o

drivers/clk/keystone/clk-am62-dss.c

@@ -0,0 +1,164 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright (C) 2023 Texas Instruments Incorporated - https://www.ti.com/
+ */
+#include <linux/module.h>
+#include <linux/clk-provider.h>
+#include <linux/platform_device.h>
+
+struct ti_am62_dss_clk {
+	struct clk_hw	hw;
+	unsigned int	div;
+};
+
+#define to_ti_am62_dss_clk(_hw) \
+	container_of(_hw, struct ti_am62_dss_clk, hw)
+
+static unsigned long ti_am62_dss_clk_recalc_rate(struct clk_hw *hw,
+						 unsigned long parent_rate)
+{
+	struct ti_am62_dss_clk *priv = to_ti_am62_dss_clk(hw);
+	unsigned long rate;
+
+	rate = parent_rate;
+	do_div(rate, priv->div);
+	return (unsigned long)rate;
+}
+
+static long ti_am62_dss_clk_round_rate(struct clk_hw *hw, unsigned long rate,
+				       unsigned long *prate)
+{
+	struct ti_am62_dss_clk *priv = to_ti_am62_dss_clk(hw);
+
+	if (clk_hw_get_flags(hw) & CLK_SET_RATE_PARENT) {
+		unsigned long best_parent;
+
+		best_parent = rate * priv->div;
+		*prate = clk_hw_round_rate(clk_hw_get_parent(hw), best_parent);
+	}
+
+	return (*prate / priv->div);
+}
+
+static int ti_am62_dss_clk_set_rate(struct clk_hw *hw, unsigned long rate,
+				    unsigned long parent_rate)
+{
+	/*
+	 * We must report success but we can do so unconditionally because
+	 * ti_am62_dss_clk_round_rate returns values that ensure this call is a
+	 * nop.
+	 */
+
+	return 0;
+}
+
+static const struct clk_ops ti_am62_dss_clk_ops = {
+	.round_rate = ti_am62_dss_clk_round_rate,
+	.set_rate = ti_am62_dss_clk_set_rate,
+	.recalc_rate = ti_am62_dss_clk_recalc_rate,
+};
+
+static struct clk_hw *
+clk_hw_register_am62_dss_clk(struct device *dev, const char *name,
+			     unsigned long flags, unsigned int div)
+{
+	struct ti_am62_dss_clk *priv;
+	struct clk_init_data init = { };
+	struct clk_parent_data pdata = { .index = 0 };
+	int ret;
+
+	priv = devm_kzalloc(dev, sizeof(*priv), GFP_KERNEL);
+	if (!priv)
+		return ERR_PTR(-ENOMEM);
+
+	/* struct ti_am62_dss_clk assignments */
+	priv->div = div;
+	priv->hw.init = &init;
+
+	init.name = name;
+	init.ops = &ti_am62_dss_clk_ops;
+	init.flags = flags;
+	init.parent_names = NULL;
+	init.parent_data = &pdata;
+	init.num_parents = 1;
+
+	ret = devm_clk_hw_register(dev, &priv->hw);
+	if (ret)
+		return ERR_PTR(ret);
+
+	return &priv->hw;
+}
+
+static void clk_hw_unregister_am62_dss_clk(struct clk_hw *hw)
+{
+	struct ti_am62_dss_clk *priv;
+
+	priv = to_ti_am62_dss_clk(hw);
+
+	clk_hw_unregister(hw);
+	kfree(priv);
+}
+
+static int ti_am62_dss_clk_remove(struct platform_device *pdev)
+{
+	struct clk_hw *hw = platform_get_drvdata(pdev);
+
+	of_clk_del_provider(pdev->dev.of_node);
+	clk_hw_unregister_am62_dss_clk(hw);
+
+	return 0;
+}
+
+static int ti_am62_dss_clk_probe(struct platform_device *pdev)
+{
+	struct clk_hw *hw;
+	const char *clk_name = pdev->name;
+	struct device *dev = &pdev->dev;
+	unsigned long flags = 0;
+	u32 div;
+	int ret;
+
+	if (of_property_read_u32(dev->of_node, "clock-div", &div)) {
+		dev_err(dev, "%s: TI AM62 DSS clock must have a clock-div property.\n",
+			__func__);
+		return -EIO;
+	}
+
+	flags |= CLK_SET_RATE_PARENT;
+
+	hw = clk_hw_register_am62_dss_clk(dev, clk_name, flags, div);
+	if (IS_ERR(hw)) {
+		dev_err(dev, "%s: failed to register %s\n", __func__, clk_name);
+		return PTR_ERR(hw);
+	}
+
+	ret = devm_of_clk_add_hw_provider(dev, of_clk_hw_simple_get, hw);
+	if (ret) {
+		clk_hw_unregister_am62_dss_clk(hw);
+		return ret;
+	}
+
+	platform_set_drvdata(pdev, hw);
+
+	return 0;
+}
+
+static const struct of_device_id ti_am62_dss_clk_ids[] = {
+	{ .compatible = "ti,am62-dss-vp0-div-clk" },
+	{ }
+};
+MODULE_DEVICE_TABLE(of, ti_am62_dss_clk_ids);
+
+static struct platform_driver ti_am62_dss_clk_driver = {
+	.driver = {
+		.name = "ti_am62_dss_clk",
+		.of_match_table = ti_am62_dss_clk_ids,
+	},
+	.probe = ti_am62_dss_clk_probe,
+	.remove = ti_am62_dss_clk_remove,
+};
+module_platform_driver(ti_am62_dss_clk_driver);
+
+MODULE_AUTHOR("Aradhya Bhatia <a-bhatia1@ti.com>");
+MODULE_DESCRIPTION("TI AM62 DSS - OLDI Fixed Clock Divider driver");
+MODULE_LICENSE("GPL");