[PATCH v3 2/5] tpm: ignore burstcount to improve tpm_tis send() performance

[Nayna Jain]

The TPM burstcount status indicates the number of bytes that can
be sent to the TPM without causing bus wait states.  Effectively,
it is the number of empty bytes in the command FIFO. Further,
some TPMs have a static burstcount, when the value remains zero
until the entire FIFO is empty.

This patch optimizes the tpm_tis_send_data() function by checking
the burstcount only once. And if the burstcount is valid, it writes
all the bytes at once, permitting wait states. The performance of a
34 byte extend on a TPM 1.2 with an 8 byte burstcount improved from
41msec to 14msec.

After this change, performance on a TPM 1.2 with an 8 byte
burstcount for 1000 extends improved from ~41sec to ~14sec.

Suggested-by: Ken Goldman <kgold@xxxxxxxxxxxxxxxxxx> in
conjunction with the TPM Device Driver work group.
Signed-off-by: Nayna Jain <nayna@xxxxxxxxxxxxxxxxxx>
Acked-by: Mimi Zohar <zohar@xxxxxxxxxxxxxxxxxx>
---
 drivers/char/tpm/tpm_tis_core.c | 42 ++++++++++++++++-------------------------
 1 file changed, 16 insertions(+), 26 deletions(-)

diff --git a/drivers/char/tpm/tpm_tis_core.c b/drivers/char/tpm/tpm_tis_core.c
index b33126a35694..8da425e1783f 100644
--- a/drivers/char/tpm/tpm_tis_core.c
+++ b/drivers/char/tpm/tpm_tis_core.c
@@ -316,7 +316,6 @@ static int tpm_tis_send_data(struct tpm_chip *chip, u8 *buf, size_t len)
 {
 	struct tpm_tis_data *priv = dev_get_drvdata(&chip->dev);
 	int rc, status, burstcnt;
-	size_t count = 0;
 	bool itpm = priv->flags & TPM_TIS_ITPM_WORKAROUND;
 
 	status = tpm_tis_status(chip);
@@ -330,35 +329,26 @@ static int tpm_tis_send_data(struct tpm_chip *chip, u8 *buf, size_t len)
 		}
 	}
 
-	while (count < len - 1) {
-		burstcnt = get_burstcount(chip);
-		if (burstcnt < 0) {
-			dev_err(&chip->dev, "Unable to read burstcount\n");
-			rc = burstcnt;
-			goto out_err;
-		}
-		burstcnt = min_t(int, burstcnt, len - count - 1);
-		rc = tpm_tis_write_bytes(priv, TPM_DATA_FIFO(priv->locality),
-					 burstcnt, buf + count);
-		if (rc < 0)
-			goto out_err;
+	/*
+	 * Get the initial burstcount to ensure TPM is ready to
+	 * accept data.
+	 */
+	burstcnt = get_burstcount(chip);
+	if (burstcnt < 0) {
+		dev_err(&chip->dev, "Unable to read burstcount\n");
+		rc = burstcnt;
+		goto out_err;
+	}
 
-		count += burstcnt;
+	burstcnt = len - 1;
 
-		if (wait_for_tpm_stat(chip, TPM_STS_VALID, chip->timeout_c,
-					&priv->int_queue, false) < 0) {
-			rc = -ETIME;
-			goto out_err;
-		}
-		status = tpm_tis_status(chip);
-		if (!itpm && (status & TPM_STS_DATA_EXPECT) == 0) {
-			rc = -EIO;
-			goto out_err;
-		}
-	}
+	rc = tpm_tis_write_bytes(priv, TPM_DATA_FIFO(priv->locality),
+			burstcnt, buf);
+	if (rc < 0)
+		goto out_err;
 
 	/* write last byte */
-	rc = tpm_tis_write8(priv, TPM_DATA_FIFO(priv->locality), buf[count]);
+	rc = tpm_tis_write8(priv, TPM_DATA_FIFO(priv->locality), buf[len-1]);
 	if (rc < 0)
 		goto out_err;
 
-- 
2.13.3