clk: samsung: Add Exynos850 clock driver stub

For now it's just a stub driver to make serial driver work. Later it
will be implemented properly.

This driver doesn't really change clocks, only registers the UART clock
as a fixed-rate clock. Without this clock driver the UART driver won't
work, as it's trying to obtain "uart" clock and fails if it's not able
to. From drivers/tty/serial/samsung_tty.c:

8<------------------------------------------------------------------->8
    ourport->clk = clk_get(&platdev->dev, "uart");
    if (IS_ERR(ourport->clk)) {
        pr_err("%s: Controller clock not found\n",
                dev_name(&platdev->dev));
        ret = PTR_ERR(ourport->clk);
        goto err;
    }
8<------------------------------------------------------------------->8

In order to get functional serial console we have to implement that
minimal clock driver with "uart" clock. It's not necessary to actually
configure clocks, as those are already configured in bootloader, so
kernel can rely on that for now.

80 column limit is broken here to make checkpatch happy, otherwise it
swears about incorrect __initconst usage.

Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>

drivers/clk/samsung/Makefile

@@ -17,6 +17,7 @@ obj-$(CONFIG_EXYNOS_ARM64_COMMON_CLK)	+= clk-exynos5433.o
 obj-$(CONFIG_EXYNOS_AUDSS_CLK_CON) += clk-exynos-audss.o
 obj-$(CONFIG_EXYNOS_CLKOUT)	+= clk-exynos-clkout.o
 obj-$(CONFIG_EXYNOS_ARM64_COMMON_CLK)	+= clk-exynos7.o
+obj-$(CONFIG_EXYNOS_ARM64_COMMON_CLK)	+= clk-exynos850.o
 obj-$(CONFIG_S3C2410_COMMON_CLK)+= clk-s3c2410.o
 obj-$(CONFIG_S3C2410_COMMON_DCLK)+= clk-s3c2410-dclk.o
 obj-$(CONFIG_S3C2412_COMMON_CLK)+= clk-s3c2412.o

drivers/clk/samsung/clk-exynos850.c

@@ -0,0 +1,63 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Copyright (C) 2019 Samsung Electronics Co., Ltd.
+ * Copyright (C) 2021 Linaro Ltd.
+ *
+ * Common Clock Framework support for Exynos850 SoC.
+ */
+
+#include <linux/clk.h>
+#include <linux/clkdev.h>
+#include <linux/clk-provider.h>
+#include <linux/of.h>
+#include <linux/of_address.h>
+#include <dt-bindings/clock/exynos850.h>
+
+#include "clk.h"
+
+/* Fixed rate clocks generated outside the SoC */
+static struct samsung_fixed_rate_clock exynos850_fixed_rate_ext_clks[] __initdata = {
+	FRATE(OSCCLK, "fin_pll", NULL, 0, 26000000),
+};
+
+/*
+ * Model the UART clock as a fixed-rate clock for now, to make serial driver
+ * work. This clock is already configured in the bootloader.
+ */
+static const struct samsung_fixed_rate_clock exynos850_peri_clks[] __initconst = {
+	FRATE(DOUT_UART, "DOUT_UART", NULL, 0, 200000000),
+};
+
+static const struct of_device_id ext_clk_match[] __initconst = {
+	{ .compatible = "samsung,exynos850-oscclk", .data = (void *)0 },
+	{},
+};
+
+void __init exynos850_clk_init(struct device_node *np)
+{
+	void __iomem *reg_base;
+	struct samsung_clk_provider *ctx;
+
+	if (!np)
+		panic("%s: unable to determine soc\n", __func__);
+
+	reg_base = of_iomap(np, 0);
+	if (!reg_base)
+		panic("%s: failed to map registers\n", __func__);
+
+	ctx = samsung_clk_init(np, reg_base, CLK_NR_CLKS);
+	if (!ctx)
+		panic("%s: unable to allocate ctx\n", __func__);
+
+	samsung_clk_of_register_fixed_ext(ctx,
+			exynos850_fixed_rate_ext_clks,
+			ARRAY_SIZE(exynos850_fixed_rate_ext_clks),
+			ext_clk_match);
+
+	samsung_clk_register_fixed_rate(ctx, exynos850_peri_clks,
+			ARRAY_SIZE(exynos850_peri_clks));
+
+	samsung_clk_of_add_provider(np, ctx);
+}
+
+CLK_OF_DECLARE(exynos850_clk, "samsung,exynos850-clock", exynos850_clk_init);