ARM: bcm2835: implement machine restart hook

Implement the machine restart hook using the SoC's watchdog timer module.
To support this, define a DT binding for the watchdog module, and add it
to the device tree.

The downstream rpi-split branch contains a full watchdog timer driver
implementation, which also implements the restart hook. However, the
restart function is largely separate from the watchdog driver, so for
simplicity, the restart hook is implemented here directly in the main
machine source file.

Overall structure (separate setup/restart) functions derived from the
picoxcell ARM support.

Watchdog register IO sequence taken from code by Simon Arlott. Note that
the watchdog module is not documented in BCM2835-ARM-Peripherals.pdf.
Signed-off-by: Stephen Warren <swarren@wwwdotorg.org>

Documentation/devicetree/bindings/watchdog/brcm,bcm2835-pm-wdog.txt

+BCM2835 Watchdog timer
+
+Required properties:
+
+- compatible : should be "brcm,bcm2835-pm-wdt"
+- reg : Specifies base physical address and size of the registers.
+
+Example:
+
+watchdog {
+	compatible = "brcm,bcm2835-pm-wdt";
+	reg = <0x7e100000 0x28>;
+};

arch/arm/boot/dts/bcm2835.dtsi

@@ -29,6 +29,11 @@ intc: interrupt-controller {
 			#interrupt-cells = <2>;
 		};
 
+		watchdog {
+			compatible = "brcm,bcm2835-pm-wdt";
+			reg = <0x7e100000 0x28>;
+		};
+
 		uart@20201000 {
 			compatible = "brcm,bcm2835-pl011", "arm,pl011", "arm,primecell";
 			reg = <0x7e201000 0x1000>;

arch/arm/mach-bcm2835/bcm2835.c

@@ -12,8 +12,10 @@
  * GNU General Public License for more details.
  */
 
+#include <linux/delay.h>
 #include <linux/init.h>
 #include <linux/irqchip/bcm2835.h>
+#include <linux/of_address.h>
 #include <linux/of_platform.h>
 #include <linux/bcm2835_timer.h>
 #include <linux/clk/bcm2835.h>
@@ -23,6 +25,48 @@
 
 #include <mach/bcm2835_soc.h>
 
+#define PM_RSTC				0x1c
+#define PM_WDOG				0x24
+
+#define PM_PASSWORD			0x5a000000
+#define PM_RSTC_WRCFG_MASK		0x00000030
+#define PM_RSTC_WRCFG_FULL_RESET	0x00000020
+
+static void __iomem *wdt_regs;
+
+/*
+ * The machine restart method can be called from an atomic context so we won't
+ * be able to ioremap the regs then.
+ */
+static void bcm2835_setup_restart(void)
+{
+	struct device_node *np = of_find_compatible_node(NULL, NULL,
+						"brcm,bcm2835-pm-wdt");
+	if (WARN(!np, "unable to setup watchdog restart"))
+		return;
+
+	wdt_regs = of_iomap(np, 0);
+	WARN(!wdt_regs, "failed to remap watchdog regs");
+}
+
+static void bcm2835_restart(char mode, const char *cmd)
+{
+	u32 val;
+
+	if (!wdt_regs)
+		return;
+
+	/* use a timeout of 10 ticks (~150us) */
+	writel_relaxed(10 | PM_PASSWORD, wdt_regs + PM_WDOG);
+	val = readl_relaxed(wdt_regs + PM_RSTC);
+	val &= ~PM_RSTC_WRCFG_MASK;
+	val |= PM_PASSWORD | PM_RSTC_WRCFG_FULL_RESET;
+	writel_relaxed(val, wdt_regs + PM_RSTC);
+
+	/* No sleeping, possibly atomic. */
+	mdelay(1);
+}
+
 static struct map_desc io_map __initdata = {
 	.virtual = BCM2835_PERIPH_VIRT,
 	.pfn = __phys_to_pfn(BCM2835_PERIPH_PHYS),
@@ -39,6 +83,7 @@ void __init bcm2835_init(void)
 {
 	int ret;
 
+	bcm2835_setup_restart();
 	bcm2835_init_clocks();
 
 	ret = of_platform_populate(NULL, of_default_bus_match_table, NULL,
@@ -60,5 +105,6 @@ DT_MACHINE_START(BCM2835, "BCM2835")
 	.handle_irq = bcm2835_handle_irq,
 	.init_machine = bcm2835_init,
 	.timer = &bcm2835_timer,
+	.restart = bcm2835_restart,
 	.dt_compat = bcm2835_compat
 MACHINE_END