Commit 8a55256a authored by Jarkko Sakkinen's avatar Jarkko Sakkinen
Browse files

Documentation: tpm_tis 



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.

Reviewed-by: default avatarBagas Sanjaya <bagasdotme@gmail.com>
Signed-off-by: default avatarJarkko Sakkinen <jarkko@kernel.org>
parent d14d2cc2

Documentation/security/tpm/index.rst

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@@ -5,6 +5,7 @@ Trusted Platform Module documentation 
.. toctree::



   tpm_event_log

   tpm_tis

   tpm_vtpm_proxy

   xen-tpmfront

   tpm_ftpm_tee

Documentation/security/tpm/tpm_tis.rst

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.. 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 historical reasons above the original MMIO driver is called tpm_tis and the

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/