hwrng: add per-device entropy derating

This patch introduces a derating factor to struct hwrng for
the random bits going into the kernel input pool, and a common
default derating for drivers which do not specify one.
Signed-off-by: Torsten Duwe <duwe@suse.de>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Acked-by: H. Peter Anvin <hpa@zytor.com>

drivers/char/hw_random/core.c

@@ -56,11 +56,15 @@ static LIST_HEAD(rng_list);
 static DEFINE_MUTEX(rng_mutex);
 static int data_avail;
 static u8 *rng_buffer, *rng_fillbuf;
-static unsigned short current_quality = 700; /* an arbitrary 70% */
+static unsigned short current_quality;
+static unsigned short default_quality; /* = 0; default to "off" */
 
 module_param(current_quality, ushort, 0644);
 MODULE_PARM_DESC(current_quality,
 		 "current hwrng entropy estimation per mill");
+module_param(default_quality, ushort, 0644);
+MODULE_PARM_DESC(default_quality,
+		 "default entropy content of hwrng per mill");
 
 static void start_khwrngd(void);
 
@@ -79,6 +83,11 @@ static inline int hwrng_init(struct hwrng *rng)
 			return err;
 	}
 
+	current_quality = rng->quality ? : default_quality;
+	current_quality &= 1023;
+
+	if (current_quality == 0 && hwrng_fill)
+		kthread_stop(hwrng_fill);
 	if (current_quality > 0 && !hwrng_fill)
 		start_khwrngd();
 

include/linux/hw_random.h

@@ -29,6 +29,8 @@
  * @read:		New API. drivers can fill up to max bytes of data
  *			into the buffer. The buffer is aligned for any type.
  * @priv:		Private data, for use by the RNG driver.
+ * @quality:		Estimation of true entropy in RNG's bitstream
+ *			(per mill).
  */
 struct hwrng {
 	const char *name;
@@ -38,6 +40,7 @@ struct hwrng {
 	int (*data_read)(struct hwrng *rng, u32 *data);
 	int (*read)(struct hwrng *rng, void *data, size_t max, bool wait);
 	unsigned long priv;
+	unsigned short quality;
 
 	/* internal. */
 	struct list_head list;