clk: add stub clock driver

Add a stub clock driver which can be used to bind clock controllers
which aren't required for the platform to boot, but which are needed for
U-Boot drivers to work.

In addition, add a NOP parent driver to allow for binding the parent
nodes of the clock.

Initially this driver supports a Qualcomm platform where the MMC driver
tries to fetch the RPM clock controller, which is not actually required
for the device to work.

Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Acked-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Signed-off-by: Caleb Connolly <caleb.connolly@linaro.org>

drivers/clk/Kconfig

@@ -96,6 +96,13 @@ config SPL_CLK_GPIO
 	  Enable this option to add GPIO-controlled clock gate driver
 	  in U-Boot SPL.
 
+config CLK_STUB
+	bool "Stub clock driver"
+	depends on CLK
+	help
+	  Enable this to provide a stub clock driver for non-essential clock
+	  controllers.
+
 config CLK_BCM6345
 	bool "Clock controller driver for BCM6345"
 	depends on CLK && ARCH_BMIPS

drivers/clk/Makefile

@@ -11,6 +11,7 @@ obj-$(CONFIG_$(PHASE_)CLK_CCF) += clk.o clk-divider.o clk-mux.o clk-gate.o
 obj-$(CONFIG_$(PHASE_)CLK_CCF) += clk-fixed-factor.o
 obj-$(CONFIG_$(PHASE_)CLK_COMPOSITE_CCF) += clk-composite.o
 obj-$(CONFIG_$(PHASE_)CLK_GPIO) += clk-gpio.o
+obj-$(CONFIG_$(PHASE_)CLK_STUB) += clk-stub.o
 
 obj-y += adi/
 obj-y += analogbits/

drivers/clk/clk-stub.c

@@ -0,0 +1,66 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Stub clk driver for non-essential clocks.
+ *
+ * This driver should be used for clock controllers
+ * which are described as dependencies in DT but aren't
+ * actually necessary for hardware functionality.
+ */
+
+#include <clk-uclass.h>
+#include <dm.h>
+
+/* NOP parent nodes to stub clocks */
+static const struct udevice_id nop_parent_ids[] = {
+	{ .compatible = "qcom,rpm-proc" },
+	{ .compatible = "qcom,glink-rpm" },
+	{ .compatible = "qcom,rpm-sm6115" },
+	{ }
+};
+
+U_BOOT_DRIVER(nop_parent) = {
+	.name = "nop_parent",
+	.id = UCLASS_NOP,
+	.of_match = nop_parent_ids,
+	.bind = dm_scan_fdt_dev,
+	.flags = DM_FLAG_DEFAULT_PD_CTRL_OFF,
+};
+
+static ulong stub_clk_set_rate(struct clk *clk, ulong rate)
+{
+	return (clk->rate = rate);
+}
+
+static ulong stub_clk_get_rate(struct clk *clk)
+{
+	return clk->rate;
+}
+
+static int stub_clk_nop(struct clk *clk)
+{
+	return 0;
+}
+
+static struct clk_ops stub_clk_ops = {
+	.set_rate = stub_clk_set_rate,
+	.get_rate = stub_clk_get_rate,
+	.enable = stub_clk_nop,
+	.disable = stub_clk_nop,
+};
+
+static const struct udevice_id stub_clk_ids[] = {
+	{ .compatible = "qcom,rpmcc" },
+	{ .compatible = "qcom,sm8250-rpmh-clk" },
+	{ .compatible = "qcom,sm8550-rpmh-clk" },
+	{ .compatible = "qcom,sm8650-rpmh-clk" },
+	{ }
+};
+
+U_BOOT_DRIVER(clk_stub) = {
+	.name = "clk_stub",
+	.id = UCLASS_CLK,
+	.ops = &stub_clk_ops,
+	.of_match = stub_clk_ids,
+	.flags = DM_FLAG_DEFAULT_PD_CTRL_OFF,
+};
+