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SubjectRe: [PATCH v2 1/4] tpm: ignore burstcount to improve tpm_tis send() performance.
On Wed, Sep 06, 2017 at 08:56:36AM -0400, Nayna Jain wrote:

> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index 4e303be83df6..3c59bb91e1ee 100644
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -1465,6 +1465,14 @@
> mode generally follows that for the NaN encoding,
> except where unsupported by hardware.
>
> + ignore_burst_count [TPM_TIS_CORE]
> + tpm_tis_core driver queries for the burstcount before
> + every send call in a loop. However, it causes delay to
> + the send command for TPMs with low burstcount value.
> + Setting this value to 1, will make driver to query for
> + burstcount only once in the loop to improve the
> + performance. By default, its value is set to 0.

Really don't want to see a kernel command line parameter for this..

Please figure out a different approach or at least a better name..

> + /*
> + * Get the initial burstcount to ensure TPM is ready to
> + * accept data, even when waiting for burstcount is disabled.
> + */
> 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);
> +
> + if (ignore_burst_count)
> + sendcnt = len - 1;
> + else
> + sendcnt = min_t(int, burstcnt, len - count - 1);
> +
> rc = tpm_tis_write_bytes(priv, TPM_DATA_FIFO(priv->locality),
> - burstcnt, buf + count);
> + sendcnt, buf + count);

The problem with this approach is that the TPM could totally block the CPU for
very long periods of time.

It seems very risky to enable..

Jason

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