To allow this device to fill the kernel's entropy pool at boot,
setup a default quality for the hwrng found in Cr50.
After some testing with rngtest and dieharder it was, in short,
discovered that the RNG produces fair quality randomness, giving
around 99.93% successes in rngtest FIPS140-2.
Notably, though, when testing with dieharder it was noticed that
we get 3 WEAK results over 114, which isn't optimal, and also
the p-values distribution wasn't uniform in all the cases, so a
conservative quality value was chosen by applying an arbitrary
penalty to the calculated values.
For reference, this is how the values were calculated:
The dieharder results were averaged, then normalized (0-1000)
and re-averaged with the rngtest result (where the result was
given a score of 99.93% of 1000, so 999.3), then aggregated
together and averaged again.
An arbitrary penalty of -100 was applied due to the retrieved
value, which brings us finally to 700.
Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@xxxxxxxxxxxxx>
---
drivers/char/tpm/tpm_tis_spi_cr50.c | 4 ++++
1 file changed, 4 insertions(+)
diff --git a/drivers/char/tpm/tpm_tis_spi_cr50.c b/drivers/char/tpm/tpm_tis_spi_cr50.c
index ea759af25634..dae98dbeeeac 100644
--- a/drivers/char/tpm/tpm_tis_spi_cr50.c
+++ b/drivers/char/tpm/tpm_tis_spi_cr50.c
@@ -36,6 +36,9 @@
#define TPM_CR50_FW_VER(l) (0x0f90 | ((l) << 12))
#define TPM_CR50_MAX_FW_VER_LEN 64
+/* Default quality for hwrng. */
+#define TPM_CR50_DEFAULT_RNG_QUALITY 700
+
struct cr50_spi_phy {
struct tpm_tis_spi_phy spi_phy;
@@ -264,6 +267,7 @@ int cr50_spi_probe(struct spi_device *spi)
phy = &cr50_phy->spi_phy;
phy->flow_control = cr50_spi_flow_control;
phy->wake_after = jiffies;
+ phy->priv.rng_quality = TPM_CR50_DEFAULT_RNG_QUALITY;
init_completion(&phy->ready);
cr50_phy->access_delay = CR50_NOIRQ_ACCESS_DELAY;
--
2.33.0
