timer: Add generic driver for RISC-V privileged architecture defined timer

RISC-V privileged architecture v1.10 defines a real-time counter,
exposed as a memory-mapped machine-mode register - mtime. mtime must
run at constant frequency, and the platform must provide a mechanism
for determining the timebase of mtime. The mtime register has a
64-bit precision on all RV32, RV64, and RV128 systems.

Different platform may have different implementation of the mtime
block hence an API riscv_get_time() is required by this driver for
platform codes to hide such implementation details. For example,
on some platforms mtime is provided by the CLINT module, while on
some other platforms a simple 'rdtime' can be used to get the timer
counter.

With this timer driver the U-Boot timer functionalities like delay
works correctly now.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Lukas Auer <lukas.auer@aisec.fraunhofer.de>
Reviewed-by: Anup Patel <anup@brainfault.org>

drivers/timer/Kconfig

@@ -126,6 +126,13 @@ config OMAP_TIMER
 	help
 	  Select this to enable an timer for Omap devices.
 
+config RISCV_TIMER
+	bool "RISC-V timer support"
+	depends on TIMER && RISCV
+	help
+	  Select this to enable support for the timer as defined
+	  by the RISC-V privileged architecture spec.
+
 config ROCKCHIP_TIMER
 	bool "Rockchip timer support"
 	depends on TIMER

drivers/timer/Makefile

@@ -13,6 +13,7 @@ obj-$(CONFIG_CADENCE_TTC_TIMER)	+= cadence-ttc.o
 obj-$(CONFIG_DESIGNWARE_APB_TIMER)	+= dw-apb-timer.o
 obj-$(CONFIG_MPC83XX_TIMER) += mpc83xx_timer.o
 obj-$(CONFIG_OMAP_TIMER)	+= omap-timer.o
+obj-$(CONFIG_RISCV_TIMER) += riscv_timer.o
 obj-$(CONFIG_ROCKCHIP_TIMER) += rockchip_timer.o
 obj-$(CONFIG_SANDBOX_TIMER)	+= sandbox_timer.o
 obj-$(CONFIG_STI_TIMER)		+= sti-timer.o

drivers/timer/riscv_timer.c

@@ -0,0 +1,56 @@
+// SPDX-License-Identifier: GPL-2.0+
+/*
+ * Copyright (C) 2018, Bin Meng <bmeng.cn@gmail.com>
+ *
+ * RISC-V privileged architecture defined generic timer driver
+ *
+ * This driver relies on RISC-V platform codes to provide the essential API
+ * riscv_get_time() which is supposed to return the timer counter as defined
+ * by the RISC-V privileged architecture spec.
+ *
+ * This driver can be used in both M-mode and S-mode U-Boot.
+ */
+
+#include <common.h>
+#include <dm.h>
+#include <errno.h>
+#include <timer.h>
+#include <asm/io.h>
+
+/**
+ * riscv_get_time() - get the timer counter
+ *
+ * Platform codes should provide this API in order to make this driver function.
+ *
+ * @time:	the 64-bit timer count  as defined by the RISC-V privileged
+ *		architecture spec.
+ * @return:	0 on success, -ve on error.
+ */
+extern int riscv_get_time(u64 *time);
+
+static int riscv_timer_get_count(struct udevice *dev, u64 *count)
+{
+	return riscv_get_time(count);
+}
+
+static int riscv_timer_probe(struct udevice *dev)
+{
+	struct timer_dev_priv *uc_priv = dev_get_uclass_priv(dev);
+
+	/* clock frequency was passed from the cpu driver as driver data */
+	uc_priv->clock_rate = dev->driver_data;
+
+	return 0;
+}
+
+static const struct timer_ops riscv_timer_ops = {
+	.get_count = riscv_timer_get_count,
+};
+
+U_BOOT_DRIVER(riscv_timer) = {
+	.name = "riscv_timer",
+	.id = UCLASS_TIMER,
+	.probe = riscv_timer_probe,
+	.ops = &riscv_timer_ops,
+	.flags = DM_FLAG_PRE_RELOC,
+};