Re: [PATCH] tpm: improve tpm_tis send() performance by ignoring burstcount

Jarkko Sakkinen
2017-08-08T19:07:45Z

On Mon, Aug 07, 2017 at 07:46:32AM -0400, Nayna Jain wrote:
> 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 ignores burstcount, permitting wait states, and thus
> writes the command as fast as the TPM can accept the bytes.
> The performance of a 34 byte extend on a TPM 1.2 improved from
> 52 msec to 11 msec.
>
> Suggested-by: Ken Goldman <kgold@linux.vnet.ibm.com> in
> conjunction with the TPM Device Driver work group.
> Signed-off-by: Nayna Jain <nayna@linux.vnet.ibm.com>
> Acked-by: Mimi Zohar <zohar@linux.vnet.ibm.com>
> ---
> drivers/char/tpm/tpm_tis_core.c | 45 ++---------------------------------------
> 1 file changed, 2 insertions(+), 43 deletions(-)
>
> diff --git a/drivers/char/tpm/tpm_tis_core.c b/drivers/char/tpm/tpm_tis_core.c
> index b617b2eeb080..478cbc0f61c3 100644
> --- a/drivers/char/tpm/tpm_tis_core.c
> +++ b/drivers/char/tpm/tpm_tis_core.c
> @@ -255,9 +255,7 @@ static int tpm_tis_recv(struct tpm_chip *chip, u8 *buf, size_t count)
> 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;
> + int rc, status;

As you anyway edit that line you could turn this as:

int rc;
int status;

> status = tpm_tis_status(chip);
> if ((status & TPM_STS_COMMAND_READY) == 0) {
> @@ -270,49 +268,10 @@ 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;
> -
> - count += burstcnt;
> -
> - 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;
> - }
> - }
> -
> - /* write last byte */
> - rc = tpm_tis_write8(priv, TPM_DATA_FIFO(priv->locality), buf[count]);
> + rc = tpm_tis_write_bytes(priv, TPM_DATA_FIFO(priv->locality), len, buf);
> if (rc < 0)
> goto out_err;
>
> - 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;
> - }
> -
> return 0;
>
> out_err:
> --
> 2.13.3

Here's an open question that I do not know the answer: can ignoring
burst count cause hardware issues in the field? The commit message
does not sort it out so I don't really feel safe merging this commit.

/Jarkko