Re: [PATCH v2 1/4] tpm: ignore burstcount to improve tpm_tis send() performance.

[Jarkko Sakkinen]

On Wed, Sep 06, 2017 at 08:56:36AM -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 adds an optimization to check for burstcount only once.
> And if it 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 41 msec to 14 msec.
> 
> This functionality is enabled only by passing module
> parameter ignore_burst_count=1. By default, this parameter
> is disabled.
> 
> 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>
> ---
>  Documentation/admin-guide/kernel-parameters.txt |  8 ++++++++
>  drivers/char/tpm/tpm_tis_core.c                 | 24 +++++++++++++++++++++---
>  2 files changed, 29 insertions(+), 3 deletions(-)
> 
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index 4e303be83df6..3c59bb91e1ee 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ 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.
> +
>  	ignore_loglevel	[KNL]
>  			Ignore loglevel setting - this will print /all/
>  			kernel messages to the console. Useful for debugging.
> diff --git a/drivers/char/tpm/tpm_tis_core.c b/drivers/char/tpm/tpm_tis_core.c
> index 63bc6c3b949e..6b9bf4c4d434 100644
> --- a/drivers/char/tpm/tpm_tis_core.c
> +++ b/drivers/char/tpm/tpm_tis_core.c
> @@ -31,6 +31,11 @@
>  #include "tpm.h"
>  #include "tpm_tis_core.h"
>  
> +static bool ignore_burst_count = false;
> +module_param(ignore_burst_count, bool, 0444);
> +MODULE_PARM_DESC(ignore_burst_count,
> +		"Ignore burstcount value while writing data");
> +
>  /* Before we attempt to access the TPM we must see that the valid bit is set.
>   * The specification says that this bit is 0 at reset and remains 0 until the
>   * 'TPM has gone through its self test and initialization and has established
> @@ -256,6 +261,7 @@ 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;
> +	int sendcnt;
>  	size_t count = 0;
>  	bool itpm = priv->flags & TPM_TIS_ITPM_WORKAROUND;
>  
> @@ -271,19 +277,31 @@ static int tpm_tis_send_data(struct tpm_chip *chip, u8 *buf, size_t len)
>  	}
>  
>  	while (count < len - 1) {
> +
> +		/*
> +		 * 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);
>  		if (rc < 0)
>  			goto out_err;
>  
> -		count += burstcnt;
> +		count += sendcnt;
> +		if (ignore_burst_count)
> +			continue;
>  
>  		if (wait_for_tpm_stat(chip, TPM_STS_VALID, chip->timeout_c,
>  					&priv->int_queue, false) < 0) {
> -- 
> 2.13.3
> 

Makes sense to discuss whether to have the kernel command-line
parameter or not before applying this.

To fuel the discussion, alternative to this would be:

1. Have this always on i.e. no command-line parameter.
2. If someone yells, we add the command-line parameter later on.

/Jarkko