Re: [PATCH 2/2] Documentation: tpm_tis

["Jarkko Sakkinen" <jarkko@kernel.org>]

On Thu Apr 11, 2024 at 1:50 PM EEST, Bagas Sanjaya wrote:
> On Tue, Apr 09, 2024 at 10:08:47PM +0300, Jarkko Sakkinen wrote:
> > diff --git a/Documentation/security/tpm/tpm_tis.rst b/Documentation/security/tpm/tpm_tis.rst
> > new file mode 100644
> > index 000000000000..b448ea3db71d
> > --- /dev/null
> > +++ b/Documentation/security/tpm/tpm_tis.rst
> > @@ -0,0 +1,46 @@
> > +.. SPDX-License-Identifier: GPL-2.0
> > +
> > +=========================
> > +TPM FIFO interface driver
> > +=========================
> > +
> > +TCG PTP Specification defines two interface types: FIFO and CRB. The former is
> > +based on sequenced read and write operations,  and the latter is based on a
> > +buffer containing the full command or response.
> > +
> > +FIFO (First-In-First-Out) interface is used by the tpm_tis_core dependent
> > +drivers. Originally Linux had only a driver called tpm_tis, which covered
> > +memory mapped (aka MMIO) interface but it was later on extended to cover other
> > +physical interfaces supported by the TCG standard.
> > +
> > +For legacy compliance the original MMIO driver is called tpm_tis and the
> Did you mean "For historical reasons above ..."?

That would be better wording.

> > +framework for FIFO drivers is named as tpm_tis_core. The postfix "tis" in
> > +tpm_tis comes from the TPM Interface Specification, which is the hardware
> > +interface specification for TPM 1.x chips.
> > +
> > +Communication is based on a 20 KiB buffer shared by the TPM chip through a
> > +hardware bus or memory map, depending on the physical wiring. The buffer is
> > +further split into five equal-size 4 KiB buffers, which provide equivalent
> > +sets of 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:
> > +
> > +- 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.
> > +
> > +References
> > +==========
> > +
> > +TCG PC Client Platform TPM Profile (PTP) Specification
> > +https://trustedcomputinggroup.org/resource/pc-client-platform-tpm-profile-ptp-specification/
>
> Other than that,
>
> Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com>


Thanks! I'll apply this with the fix you proposed.

For everyone: this is by no means perfect. The point is to seed
something we can build on top of. So I leave it rather lacking stuff
than try to document every possible bells and whistle. This can be
then improved based on discussions and future patch sets.

BR, Jarkko