[PATCH] Documentation: tpm_tis

[Jarkko Sakkinen <jarkko@kernel.org>]

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
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>
---
 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..3cec0216a169
--- /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
+`tpm_tis_core` dependent drivers. The prefix "tis" is named after TPM
+Interface Specification, which is the hardware interface specification for
+TPM 1.x chips.
+
+Communication is based on a 5 KiB buffer shared by the TPM chip through a
+hardware bus or memory map. The buffer is further split to five equal size
+buffers, which provide equivalent sets of registers for communication
+between CPU and TPM. The communication end points are called *localities*
+in the TCG terminology.
+
+When a kernel wants to send a commands to the TPM chip, it first reserves
+locality 0 by setting `requestUse` bit in `TPM_ACCESS` register. The bit is
+cleared by the chip when the access is granted. Once completed its
+communication, it sets `activeLocity` bit in the same register.
+
+Pending localities are served in order by the chip descending orderm and
+one at a time:
+
+- Locality 0 has the lowest priority.
+- Locality 5 has the highest priotiy.
+
+Further information on purpose and meaning of the localities can be found
+from section 3.2 of TCG PC Client Platform TPM Profile Specification.
-- 
2.40.1