Re: [PATCH 2/2] reset: Add Infineon SLB9670 TPM reset driver

Alexander Steffen
2023-12-21T10:11:15Z

On 18.12.2023 18:51, Lukas Wunner wrote:
> Hi Francesco,
>
> On Mon, Dec 18, 2023 at 06:34:00PM +0100, Francesco Dolcini wrote:
>> On Thu, Nov 23, 2023 at 09:59:43AM +0100, Lukas Wunner wrote:
>>> On Wed, Nov 22, 2023 at 04:15:18PM +0100, Francesco Dolcini wrote:
>>>> On Wed, Nov 22, 2023 at 12:29:49PM +0100, Lukas Wunner wrote:
>>>>> On Wed, Nov 22, 2023 at 12:33:58AM +0100, Francesco Dolcini wrote:
>>>>>> Not to mention that I was able to see the driver probe succeed in a
>>>>>> similar setup to the one you are describing in the commit message
>>>>>> (different board, arm64, but nothing done by the platform firmware).
>>>>>
>>>>> Hm, is the RST# pin even connected on that board?
>>>>
>>>> Yes, it's connected and it is asserted/de-asserted (aka toggled) during
>>>> startup from the HW reset circuit. However this is not implementing the
>>>> reset sequence you are implementing here.
>>>
>>> Section 4.5 of the datasheet seems to indicate that unless the sequence
>>> in Figure 3 is observed, the TPM may enter a defense mode against
>>> dictionary attacks "from which a recovery is very complex or even not
>>> possible."
>>>
>>> Simply toggling the RST# pin might therefore not be sufficient to ensure
>>> the TPM is operable.
>>
>> I am trying to follow-up with infineon on this regard, do you already
>> have any insight from them maybe?
>>
>> Maybe this procedure is relevant only when the device is in "security
>> defense state"?
>
> Sorry, I honestly don't know. A colleague has talked to an FAE at an
> Infineon reseller but they couldn't give a definitive answer either.
> I'm very interested to hear whatever you learn from Infineon.

Infineon is here :)

I'm sorry, the document is a little confusing, we'll fix that in the
future. What the document wants to say is this: Any time you assert
RST#, the TPM will reset. But if you reset the TPM at certain points in
time, you will trigger some security functions. In general, as long as
it only happens occasionally, this is not a problem (you can't avoid all
power outages). Only if you happen to frequently issue resets (e.g. if
your reset pin is not a dedicated TPM reset pin but is also used for
other things), then you should make sure to wait at least t_RSTIN
between those resets (and avoid interrupting TPM command execution).

So in your case, you probably don't need to do anything special: Just
assert RST# once and the TPM will reset. This should work with basically
any TPM, so there is no need for a dedicated SLB9670 reset driver.

Alexander