linux-kernel - Re: [PATCH v2] Documentation: tpm

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Date: Wed, 20 Mar 2024 12:15:56 -0400
From: Stefan Berger <stefanb@...ux.ibm.com>
To: Jarkko Sakkinen <jarkko@...nel.org>, linux-integrity@...r.kernel.org
Cc: Jonathan Corbet <corbet@....net>,
        "Daniel P . Smith" <dpsmith@...rtussolutions.com>,
        Lino Sanfilippo <l.sanfilippo@...bus.com>,
        Jason Gunthorpe <jgg@...pe.ca>, Peter Huewe <peterhuewe@....de>,
        James Bottomley <James.Bottomley@...senPartnership.com>,
        Alexander Steffen <Alexander.Steffen@...ineon.com>,
        keyrings@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, Randy Dunlap <rdunlap@...radead.org>
Subject: Re: [PATCH v2] Documentation: tpm_tis



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@....net>
> CC: Daniel P. Smith <dpsmith@...rtussolutions.com>
> Cc: Lino Sanfilippo <l.sanfilippo@...bus.com>
> Cc: Jason Gunthorpe <jgg@...pe.ca>
> Cc: Peter Huewe <peterhuewe@....de>
> Cc: James Bottomley <James.Bottomley@...senPartnership.com>
> Cc: Alexander Steffen <Alexander.Steffen@...ineon.com>
> Cc: keyrings@...r.kernel.org
> Cc: linux-doc@...r.kernel.org
> Cc: linux-kernel@...r.kernel.org
> Cc: linux-integrity@...r.kernel.org
> Cc: Randy Dunlap <rdunlap@...radead.org>
> Signed-off-by: Jarkko Sakkinen <jarkko@...nel.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'.

> +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'.

> +Specification, which is the hardware interface specification for TPM 1.x chips.

It's also available for TPM2.

> +
> +Communication is based on a 5 KiB buffer shared by the TPM chip through a

I thought it was typically 4 KiB.

> +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

equal-sized MMIO regions?

> +registers for communication between the CPU and TPM. These communication
> +endpoints are called localities in the TCG terminology.
> +
> +When the kernel wants to send commands to the TPM chip, it first reserves
> +locality 0 by setting the requestUse bit in the TPM_ACCESS register. The bit is
> +cleared by the chip when the access is granted. Once it completes its
> +communication, the kernel writes the TPM_ACCESS.activeLocality bit. This
> +informs the chip that the locality has been relinquished.
> +
> +Pending localities are served in order by the chip in descending order, one at
> +a time:

I think I know what pending localities are because I have worked with 
this device but I am not sure whether the user can deduce this from the 
paragraph above. Also, why this particular detail when the driver only 
uses locality 0 and nobody is competing about access to localities?

> +
> +- Locality 0 has the lowest priority.
> +- Locality 5 has the highest priority.
> +
> +Further information on the purpose and meaning of the localities can be found
> +in section 3.2 of the TCG PC Client Platform TPM Profile Specification.