hwrng: core - document the quality field

quality field is currently documented as being 'per mill'.  In fact the
math involved is:

                add_hwgenerator_randomness((void *)rng_fillbuf, rc,
                                           rc * current_quality * 8 >> 10);

thus the actual definition is "bits of entropy per 1024 bits of input".

The current documentation seems to have confused multiple people
in the past, let's fix the documentation to match code.

An alternative is to change core to match driver expectations, replacing
	rc * current_quality * 8 >> 10
with
	rc * current_quality / 1000
but that has performance costs, so probably isn't a good option.

Fixes: 0f734e6e768 ("hwrng: add per-device entropy derating")
Reported-by: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

1 files changed, 2 insertions, 1 deletions

diff --git a/include/linux/hw_random.h b/include/linux/hw_random.h
index bee0827766a3..c0b93e0ff0c0 100644
--- a/include/linux/hw_random.h
+++ b/include/linux/hw_random.h
@@ -33,7 +33,8 @@
  *			and max is a multiple of 4 and >= 32 bytes.
  * @priv:		Private data, for use by the RNG driver.
  * @quality:		Estimation of true entropy in RNG's bitstream
- *			(per mill).
+ *			(in bits of entropy per 1024 bits of input;
+ *			valid values: 1 to 1024, or 0 for unknown).
  */
 struct hwrng {
 	const char *name;