[PATCH] hwrng: imx-rngc - add quality to use it as kernel entropy pool

* [PATCH] hwrng: imx-rngc - add quality to use it as kernel entropy pool
@ 2020-08-31 14:00 ` Marco Felsch
  0 siblings, 0 replies; 4+ messages in thread
From: Marco Felsch @ 2020-08-31 14:00 UTC (permalink / raw)
  To: martin, prasannatsmkumar, linux-imx, festevam, herbert, mpm,
	Anson.Huang, horia.geanta, arnd, ceggers
  Cc: linux-crypto, linux-arm-kernel, kernel

The RM describes the RNGB as follow:
8<----------------------------------------------------------------
The RNGB uses the True Random Number Generator (TRNG) and a
Pseudo-Random Number Generator (PRNG) to achieve a true randomness and
cryptographic strength.
8<----------------------------------------------------------------

The RNGB has 3 operation modes: self-test, seed-generation and the final
'random number generation' mode. Before we can retrieve random numbers
from the RNGB we need to generate the seed pool:
8<----------------------------------------------------------------
During the seed generation, the RNGB adds the entropy generated in the
TRNG to the 256-bit XKEY register. The PRNG algorithm executes 20.000
entropy samples from the TRNG to create an initial seed for the random
number generation.
8<----------------------------------------------------------------

The RNGB can generate 2^20 words (1 word == 4 byte) of 'random' data
after the seed pool was initialized. The pool needs to be reseeded if
more words are required. The reseeding is done automatically since
commit 3acd9ea9331c ("hwrng: imx-rngc - use automatic seeding").

We can't retrieve the TRNG values directly so we need a other way to get
the quality level. We know that the PRNG uses 20.000 entropy samples
from the TRNG to generate 2^20 words (1MiB) and the quality level is
defined as (in bits of entropy per 1024 bits of input). So the quality
level can be calculated by:

   20.000 * 1024
   ------------- = ~ 19.5
        2^20

Signed-off-by: Marco Felsch <m.felsch@pengutronix.de>
---
 drivers/char/hw_random/imx-rngc.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/drivers/char/hw_random/imx-rngc.c b/drivers/char/hw_random/imx-rngc.c
index 9c47e431ce90..61c844baf26e 100644
--- a/drivers/char/hw_random/imx-rngc.c
+++ b/drivers/char/hw_random/imx-rngc.c
@@ -285,6 +285,7 @@ static int imx_rngc_probe(struct platform_device *pdev)
 	rngc->rng.init = imx_rngc_init;
 	rngc->rng.read = imx_rngc_read;
 	rngc->rng.cleanup = imx_rngc_cleanup;
+	rngc->rng.quality = 19;
 
 	rngc->dev = &pdev->dev;
 	platform_set_drvdata(pdev, rngc);
-- 
2.20.1


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