linux-kernel - Re: [PATCH v2 3/4] tpm: reduce tpm_msleep() time in get

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Message-Id: <32b07575-a895-fe45-eba3-15e47adfbcac@linux.vnet.ibm.com>
Date:   Fri, 15 Sep 2017 18:10:49 +0530
From:   Nayna Jain <nayna@...ux.vnet.ibm.com>
To:     Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
Cc:     tpmdd-devel@...ts.sourceforge.net, peterhuewe@....de,
        tpmdd@...horst.net, jgunthorpe@...idianresearch.com,
        linux-security-module@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        linux-ima-devel@...ts.sourceforge.net, patrickc@...ibm.com
Subject: Re: [PATCH v2 3/4] tpm: reduce tpm_msleep() time in get_burstcount()



On 09/13/2017 06:30 AM, Jarkko Sakkinen wrote:
> On Wed, Sep 06, 2017 at 08:56:38AM -0400, Nayna Jain wrote:
>> Currently, get_burstcount() function sleeps for 5msec in a loop
>> before retrying for next query to burstcount. However, if it takes
>> lesser time for TPM to return, this 5 msec delay is longer
>> than necessary.
>>
>> This patch replaces the tpm_msleep time from 5msec to 1msec.
>>
>> After this change, performance on a TPM 1.2 with an 8 byte
>> burstcount for 1000 extends improved from ~10sec to ~9sec.
>>
>> Signed-off-by: Nayna Jain <nayna@...ux.vnet.ibm.com>
>> Acked-by: Mimi Zohar <zohar@...ux.vnet.ibm.com>
>> ---
>>   drivers/char/tpm/tpm_tis_core.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/drivers/char/tpm/tpm_tis_core.c b/drivers/char/tpm/tpm_tis_core.c
>> index d1eab29cb447..d710bbc4608b 100644
>> --- a/drivers/char/tpm/tpm_tis_core.c
>> +++ b/drivers/char/tpm/tpm_tis_core.c
>> @@ -169,7 +169,7 @@ static int get_burstcount(struct tpm_chip *chip)
>>   		burstcnt = (value >> 8) & 0xFFFF;
>>   		if (burstcnt)
>>   			return burstcnt;
>> -		tpm_msleep(TPM_TIMEOUT);
>> +		tpm_msleep(1);
>>   	} while (time_before(jiffies, stop));
>>   	return -EBUSY;
>>   }
>> -- 
>> 2.13.3
> How did you pick 1 ms delay? Should there be a constant defining it?

As per ddwg input, the command may not take more than a few
microseconds. The minimum tpm_msleep() value is 1 msec, so we really
don't have a choice.  (We're working on a patch set to lower this
value even more.)

Thanks & Regards,
     - Nayna

>
> /Jarkko
>