Re: Aw: Re: [tpmdd-devel] [PATCH] tpm: improve tpm_tis send() performance by ignoring burstcount

Ken Goldman
2017-08-11T21:53:46Z

On 8/9/2017 4:43 PM, Peter Huewe wrote:
>
> Yes that's bad, especially with current msleep(5) is actually
> msleep(20). However, please also keep in mind SPI tpms show a much
> higher burst count value, (255) our I2C TPM SLB9645 even shows
> something in the range of 1k. :)

One of our platforms has a TPM 1.2 with an 8 byte FIFO and a static
burst count. This is where we're debugging.

>> Would another solution be to reduce the burst count poll and sleep
>> to, e.g., 100 usec or even 10 usec? This would probably help
>> greatly, but till not incur the wait states that triggered the
>> NACK.
>
> If you use sleep it's not guaranteed that you wakeup after exactly xx
> specified amount of time. Just that you sleep at least xx amount of
> time. Otherwise you would have to do delay/busywaiting.

Understood. However, if the DD maintainers will not accept ignoring
burstCount, would a short sleep (e.g., 10 usec) be acceptable.

If so, we can benchmark it.

> Imho the best option is to figure out whether any vendor can
> determine the "FIFO flush time" i.e. how much time does it take to
> empty the fifo and fillup the burstcount and use this on the lower
> bound of an usleep range. I don't think tpms are in the range of < 10
> us...
>
> @Ken: Maybe can you check in DDWG?

I asked this week.

Nuvoton, ST Micro, and Infineon confirmed that the TPM empties a byte
from the FIFO in under 1 usec. So, even with a static burst count, the
entire FIFO would empty in under 10 usec.