47 lines
2.0 KiB
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47 lines
2.0 KiB
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.. SPDX-License-Identifier: GPL-2.0
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=========================
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TPM FIFO interface driver
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=========================
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TCG PTP Specification defines two interface types: FIFO and CRB. The former is
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based on sequenced read and write operations, and the latter is based on a
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buffer containing the full command or response.
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FIFO (First-In-First-Out) interface is used by the tpm_tis_core dependent
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drivers. Originally Linux had only a driver called tpm_tis, which covered
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memory mapped (aka MMIO) interface but it was later on extended to cover other
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physical interfaces supported by the TCG standard.
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For historical reasons above the original MMIO driver is called tpm_tis and the
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framework for FIFO drivers is named as tpm_tis_core. The postfix "tis" in
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tpm_tis comes from the TPM Interface Specification, which is the hardware
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interface specification for TPM 1.x chips.
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Communication is based on a 20 KiB buffer shared by the TPM chip through a
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hardware bus or memory map, depending on the physical wiring. The buffer is
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further split into five equal-size 4 KiB buffers, which provide equivalent
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sets of registers for communication between the CPU and TPM. These
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communication endpoints are called localities in the TCG terminology.
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When the kernel wants to send commands to the TPM chip, it first reserves
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locality 0 by setting the requestUse bit in the TPM_ACCESS register. The bit is
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cleared by the chip when the access is granted. Once it completes its
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communication, the kernel writes the TPM_ACCESS.activeLocality bit. This
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informs the chip that the locality has been relinquished.
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Pending localities are served in order by the chip in descending order, one at
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a time:
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- Locality 0 has the lowest priority.
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- Locality 5 has the highest priority.
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Further information on the purpose and meaning of the localities can be found
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in section 3.2 of the TCG PC Client Platform TPM Profile Specification.
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References
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==========
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TCG PC Client Platform TPM Profile (PTP) Specification
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https://trustedcomputinggroup.org/resource/pc-client-platform-tpm-profile-ptp-specification/
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