[PATCH v8 04/12] tpm: Change tpm_get_random() opportunistic

[Jarkko Sakkinen <jarkko@kernel.org>]

hwrng framework does not have a requirement that the all bytes requested
need to be provided. By enforcing such a requirement internally, TPM driver
can cause unpredictability in latency, as a single tpm_get_random() call
can result multiple TPM commands.

Especially, when TCG_TPM2_HMAC is enabled, extra roundtrips could have
significant effect to the system latency.

Thus, send TPM command only once and return bytes received instead of
committing to the number of requested bytes.

Cc: David S. Miller <davem@davemloft.net>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Cc: Eric Biggers <ebiggers@kernel.org>
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>
---
v7:
- Given that hwrng is now only caller for tpm_get_random(), remove the
  wait parameter.
v4:
- Fixed grammar mistakes.
---
 drivers/char/tpm/tpm-interface.c | 28 +++++-----------------------
 1 file changed, 5 insertions(+), 23 deletions(-)

diff --git a/drivers/char/tpm/tpm-interface.c b/drivers/char/tpm/tpm-interface.c
index d157be738612..677dcef05dfb 100644
--- a/drivers/char/tpm/tpm-interface.c
+++ b/drivers/char/tpm/tpm-interface.c
@@ -626,10 +626,6 @@ static int tpm2_get_random(struct tpm_chip *chip, u8 *out, size_t max)
  */
 int tpm_get_random(struct tpm_chip *chip, u8 *out, size_t max)
 {
-	u32 num_bytes = max;
-	u8 *out_ptr = out;
-	int retries = 5;
-	int total = 0;
 	int rc;
 
 	if (!out || !max || max > TPM_MAX_RNG_DATA)
@@ -646,28 +642,14 @@ int tpm_get_random(struct tpm_chip *chip, u8 *out, size_t max)
 		rc = tpm2_start_auth_session(chip);
 		if (rc)
 			return rc;
-	}
-
-	do {
-		if (chip->flags & TPM_CHIP_FLAG_TPM2)
-			rc = tpm2_get_random(chip, out_ptr, num_bytes);
-		else
-			rc = tpm1_get_random(chip, out_ptr, num_bytes);
-
-		if (rc < 0)
-			goto err;
-
-		out_ptr += rc;
-		total += rc;
-		num_bytes -= rc;
-	} while (retries-- && total < max);
 
-	tpm_put_ops(chip);
-	return total ? total : -EIO;
+		rc = tpm2_get_random(chip, out, max);
+	} else {
+		rc = tpm1_get_random(chip, out, max);
+	}
 
-err:
 	tpm_put_ops(chip);
-	return rc;
+	return rc != 0 ? rc : -EIO;
 }
 EXPORT_SYMBOL_GPL(tpm_get_random);
 
-- 
2.39.5