rtc: wm831x: Feed the write counter into device_add_randomness()

commit 9dccf55f4cb011a7552a8a2749a580662f5ed8ed upstream.

The tamper evident features of the RTC include the "write counter" which
is a pseudo-random number regenerated whenever we set the RTC. Since this
value is unpredictable it should provide some useful seeding to the random
number generator.

Only do this on boot since the goal is to seed the pool rather than add
useful entropy.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/rtc/rtc-wm831x.c b/drivers/rtc/rtc-wm831x.c
index 59c6245e0421f21a70548108a2607ebdaa4bbd33..ea5c6f857ca5db35ebbfd3c581b0d5e7e32050d4 100644 (file)
--- a/drivers/rtc/rtc-wm831x.c
+++ b/drivers/rtc/rtc-wm831x.c
@@ -24,7 +24,7 @@
 #include <linux/mfd/wm831x/core.h>
 #include <linux/delay.h>
 #include <linux/platform_device.h>
-
+#include <linux/random.h>
 
 /*
  * R16416 (0x4020) - RTC Write Counter
@@ -96,6 +96,26 @@ struct wm831x_rtc {
        unsigned int alarm_enabled:1;
 };
 
+static void wm831x_rtc_add_randomness(struct wm831x *wm831x)
+{
+       int ret;
+       u16 reg;
+
+       /*
+        * The write counter contains a pseudo-random number which is
+        * regenerated every time we set the RTC so it should be a
+        * useful per-system source of entropy.
+        */
+       ret = wm831x_reg_read(wm831x, WM831X_RTC_WRITE_COUNTER);
+       if (ret >= 0) {
+               reg = ret;
+               add_device_randomness(&reg, sizeof(reg));
+       } else {
+               dev_warn(wm831x->dev, "Failed to read RTC write counter: %d\n",
+                        ret);
+       }
+}
+
 /*
  * Read current time and date in RTC
  */
@@ -431,6 +451,8 @@ static int wm831x_rtc_probe(struct platform_device *pdev)
                        alm_irq, ret);
        }
 
+       wm831x_rtc_add_randomness(wm831x);
+
        return 0;
 
 err: