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: Bagas Sanjaya <[e-mail]>
Signed-off-by: Jarkko Sakkinen <[e-mail]>

Documentation/security/tpm/index.rst

···
 .. toctree::
 
    tpm_event_log
+   tpm_tis
    tpm_vtpm_proxy
    xen-tpmfront
    tpm_ftpm_tee

Documentation/security/tpm/tpm_tis.rst

···
+.. 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/