xen/rpi4: implement watchdog-based reset

The preferred method to reboot RPi4 is PSCI. If it is not available,
touching the watchdog is required to be able to reboot the board.

The implementation is based on
drivers/watchdog/bcm2835_wdt.c:__bcm2835_restart in Linux v5.9-rc7.

Signed-off-by: Stefano Stabellini <stefano.stabellini@xilinx.com>
Acked-by: Julien Grall <jgrall@amazon.com>
Reviewed-by: Bertrand Marquis <bertrand.marquis@arm.com>
Tested-by: Roman Shaposhnik <roman@zededa.com>
CC: roman@zededa.com

xen/arch/arm/platforms/brcm-raspberry-pi.c

@@ -17,6 +17,10 @@
  * GNU General Public License for more details.
  */
 
+#include <xen/delay.h>
+#include <xen/mm.h>
+#include <xen/vmap.h>
+#include <asm/io.h>
 #include <asm/platform.h>
 
 static const char *const rpi4_dt_compat[] __initconst =
@@ -37,12 +41,69 @@ static const struct dt_device_match rpi4_blacklist_dev[] __initconst =
      * The aux peripheral also shares a page with the aux UART.
      */
     DT_MATCH_COMPATIBLE("brcm,bcm2835-aux"),
+    /* Special device used for rebooting */
+    DT_MATCH_COMPATIBLE("brcm,bcm2835-pm"),
     { /* sentinel */ },
 };
 
+
+#define PM_PASSWORD                 0x5a000000
+#define PM_RSTC                     0x1c
+#define PM_WDOG                     0x24
+#define PM_RSTC_WRCFG_FULL_RESET    0x00000020
+#define PM_RSTC_WRCFG_CLR           0xffffffcf
+
+static void __iomem *rpi4_map_watchdog(void)
+{
+    void __iomem *base;
+    struct dt_device_node *node;
+    paddr_t start, len;
+    int ret;
+
+    node = dt_find_compatible_node(NULL, NULL, "brcm,bcm2835-pm");
+    if ( !node )
+        return NULL;
+
+    ret = dt_device_get_address(node, 0, &start, &len);
+    if ( ret )
+    {
+        printk("Cannot read watchdog register address\n");
+        return NULL;
+    }
+
+    base = ioremap_nocache(start & PAGE_MASK, PAGE_SIZE);
+    if ( !base )
+    {
+        printk("Unable to map watchdog register!\n");
+        return NULL;
+    }
+
+    return base;
+}
+
+static void rpi4_reset(void)
+{
+    uint32_t val;
+    void __iomem *base = rpi4_map_watchdog();
+
+    if ( !base )
+        return;
+
+    /* use a timeout of 10 ticks (~150us) */
+    writel(10 | PM_PASSWORD, base + PM_WDOG);
+    val = readl(base + PM_RSTC);
+    val &= PM_RSTC_WRCFG_CLR;
+    val |= PM_PASSWORD | PM_RSTC_WRCFG_FULL_RESET;
+    writel(val, base + PM_RSTC);
+
+    /* No sleeping, possibly atomic. */
+    mdelay(1);
+}
+
 PLATFORM_START(rpi4, "Raspberry Pi 4")
     .compatible     = rpi4_dt_compat,
     .blacklist_dev  = rpi4_blacklist_dev,
+    .reset = rpi4_reset,
     .dma_bitsize    = 30,
 PLATFORM_END