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>

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;