rtc: honor device tree /alias entries when assigning IDs

Assign RTC device IDs based on device tree /aliases entries if present,
falling back to the existing numbering scheme if there is no /aliases
entry (which includes when the system isn't booted using DT), or there is
a numbering conflict.

This is useful in systems with multiple RTC devices, to ensure that the
best RTC device is selected as /dev/rtc0, which provides the overall
system time.

For example, Tegra has an on-SoC RTC that is not battery backed, typically
coupled with an off-SoC RTC that is battery backed.  Only the latter is
useful for populating the system time, yet the former is useful e.g.  for
wakeup timing, since the time is not lost when the system is sleeps.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/drivers/rtc/class.c b/drivers/rtc/class.c
index 02426812bebc5c5cc74cba4db4395e2c67d78f25..7989b5d030cb9aa23b0fe9215d1771897aae484f 100644 (file)
--- a/drivers/rtc/class.c
+++ b/drivers/rtc/class.c
@@ -14,6 +14,7 @@
 #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
 
 #include <linux/module.h>
+#include <linux/of.h>
 #include <linux/rtc.h>
 #include <linux/kdev_t.h>
 #include <linux/idr.h>
@@ -157,12 +158,25 @@ struct rtc_device *rtc_device_register(const char *name, struct device *dev,
 {
        struct rtc_device *rtc;
        struct rtc_wkalrm alrm;
-       int id, err;
+       int of_id, id = -1, err;
+
+       if (dev->of_node) {
+               of_id = of_alias_get_id(dev->of_node, "rtc");
+               if (of_id >= 0) {
+                       id = ida_simple_get(&rtc_ida, of_id, of_id + 1,
+                                           GFP_KERNEL);
+                       if (id < 0)
+                               dev_warn(dev, "/aliases ID %d not available\n",
+                                        of_id);
+               }
+       }
 
-       id = ida_simple_get(&rtc_ida, 0, 0, GFP_KERNEL);
        if (id < 0) {
-               err = id;
-               goto exit;
+               id = ida_simple_get(&rtc_ida, 0, 0, GFP_KERNEL);
+               if (id < 0) {
+                       err = id;
+                       goto exit;
+               }
        }
 
        rtc = kzalloc(sizeof(struct rtc_device), GFP_KERNEL);