Re: [PATCH v2] Documentation: tpm_tis

[Stefan Berger <stefanb@linux.ibm.com>]

On 3/21/24 11:51, Jarkko Sakkinen wrote:
> On Wed Mar 20, 2024 at 6:15 PM EET, Stefan Berger wrote:
>>
>>
>> On 3/20/24 04:56, Jarkko Sakkinen wrote:
>>> Based recent discussions on LKML, provide preliminary bits of tpm_tis_core
>>> dependent drivers. Includes only bare essentials but can be extended later
>>> on case by case. This way some people may even want to read it later on.
>>>
>>> Cc: Jonathan Corbet <corbet@lwn.net>
>>> CC: Daniel P. Smith <dpsmith@apertussolutions.com>
>>> Cc: Lino Sanfilippo <l.sanfilippo@kunbus.com>
>>> Cc: Jason Gunthorpe <jgg@ziepe.ca>
>>> Cc: Peter Huewe <peterhuewe@gmx.de>
>>> Cc: James Bottomley <James.Bottomley@HansenPartnership.com>
>>> Cc: Alexander Steffen <Alexander.Steffen@infineon.com>
>>> Cc: keyrings@vger.kernel.org
>>> Cc: linux-doc@vger.kernel.org
>>> Cc: linux-kernel@vger.kernel.org
>>> Cc: linux-integrity@vger.kernel.org
>>> Cc: Randy Dunlap <rdunlap@infradead.org>
>>> Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>
>>> ---
>>> v2:
>>> - Fixed errors reported by Randy:
>>>     https://lore.kernel.org/all/aed28265-d677-491a-a045-24b351854b24@infradead.org/
>>> - Improved the text a bit to have a better presentation.
>>> ---
>>>    Documentation/security/tpm/index.rst   |  1 +
>>>    Documentation/security/tpm/tpm_tis.rst | 30 ++++++++++++++++++++++++++
>>>    2 files changed, 31 insertions(+)
>>>    create mode 100644 Documentation/security/tpm/tpm_tis.rst
>>>
>>> diff --git a/Documentation/security/tpm/index.rst b/Documentation/security/tpm/index.rst
>>> index fc40e9f23c85..f27a17f60a96 100644
>>> --- a/Documentation/security/tpm/index.rst
>>> +++ b/Documentation/security/tpm/index.rst
>>> @@ -5,6 +5,7 @@ Trusted Platform Module documentation
>>>    .. toctree::
>>>    
>>>       tpm_event_log
>>> +   tpm_tis
>>>       tpm_vtpm_proxy
>>>       xen-tpmfront
>>>       tpm_ftpm_tee
>>> diff --git a/Documentation/security/tpm/tpm_tis.rst b/Documentation/security/tpm/tpm_tis.rst
>>> new file mode 100644
>>> index 000000000000..b331813b3c45
>>> --- /dev/null
>>> +++ b/Documentation/security/tpm/tpm_tis.rst
>>> @@ -0,0 +1,30 @@
>>> +.. SPDX-License-Identifier: GPL-2.0
>>> +
>>> +=========================
>>> +TPM FIFO interface Driver
>>> +=========================
>>> +
>>> +FIFO (First-In-First-Out) is the name of the hardware interface used by the
>>
>> FIFO is the type. I am surprised you call it a 'name'. I would say TIS
>> is the 'name'.
> 
> It's what the official specification calls it [1].
> 
> 
>>
>>> +tpm_tis_core dependent drivers. The prefix "tis" comes from the TPM Interface
>>
>> tis is a tla -- a three letter *acronym*. You aren't using it as a 'prefix'.
> 
> I don't know what "tla" means.
> 
>>
>>> +Specification, which is the hardware interface specification for TPM 1.x chips.
>>
>> It's also available for TPM2.
>   
> Yes, but TIS is the name used by the legacy specification.


The point is that TIS is not just a TPM 1.x interface but also used for 
TPM 2.
> 
>>
>>> +
>>> +Communication is based on a 5 KiB buffer shared by the TPM chip through a
>>
>> I thought it was typically 4 KiB.
> 
> You are basing this on table 9 in [1]?

Yes. See below.

> 
>>
>>> +hardware bus or memory map, depending on the physical wiring. The buffer is
>>> +further split into five equal-size buffers, which provide equivalent sets of

If you are referring to the MMIO region between 0xfed4 0000 and 0xfed4 
4fff as a buffer then you are talking about a **20kb** MMIO region 
(0x5000) that is **split** into equal-sized MMIO regions, each having 
4kb (0x1000). Yes, that's the 4kb then but there that one is no 5kb 
'further split into five equal-sized buffers' of presumably 1kb each. 
Each locality has a 0x1000 sized MMIO region.