[1/5,RFC] Documentation: Introduce Platform Firmware Runtime Update documentation

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+.. SPDX-License-Identifier: GPL-2.0
+
+========================================================
+The Linux Platform Firmware Runtime Update and Telemetry
+========================================================
+
+According to the specification of <Management Mode Firmware Runtime Update>[1],
+certain computing systems require high Service Level Agreements (SLAs) where
+system reboot fewer firmware updates are required to deploy firmware changes
+to address bug fixes, security updates and to debug and root cause issues. This
+technology is called Intel Seamless Update. The management mode (MM),
+UEFI runtime services and ACPI services handle most of the system runtime
+functions. Changing the MM code execution during runtime is called MM Runtime
+Update. Since the "MM" acronyms might be misunderstood as "Memory Management",
+this driver uses "Platform Firmware Runtime Update"(PFRU)
+
+PFRU provides the following facilities: Performs a runtime firmware driver update
+and activate. Ability to inject firmware code at runtime, for dynamic instrumentation.
+PFRU Telemetry is a service which allows Runtime Update handler to produce telemetry
+data to upper layer OS consumer at runtime. The OS provides interfaces to let the
+users query the telemetry data via read operations. The specification specifies the
+interface and recommended policy to extract the data, the format and use are left to
+individual OEM's and BIOS implementations on what that data represents.
+
+PFRU interfaces
+=====================
+
+The user space tool manipulates on /dev/pfru/update for code injection and
+driver update. PFRU stands for Platform Firmware Runtime Update, and the /dev/pfru
+directory might be reserved for future usage.
+
+ 1. mmap the capsule file
+    fd_capsule = open("capsule.cap", O_RDONLY);
+    fstat(fd_capsule, &stat);
+    addr = mmap(0, stat.st_size, PROT_READ, fd_capsule);
+
+ 2. Get the capability information(version control, etc) from BIOS via
+    read() and do sanity check in user space.
+    fd_update = open("/dev/pfru/update", O_RDWR);
+    read(fd_update, &cap, sizeof(cap));
+    sanity_check(&cap);
+
+ 3. Write the capsule file to runtime update communication buffer
+    //kernel might return error if capsule file size is longer than
+    //communication buffer
+    write(fd_update, addr, stat.st_size);
+
+ 4. Stage the code injection
+    ioctl(fd_update, PFRU_IOC_STATGE);
+
+ 5. Activate the code injection
+    ioctl(fd_update, PFRU_IOC_ACTIVATE);
+
+ 6. Stage and activate the code injection
+    ioctl(fd_update, PFRU_IOC_STAGE_ACTIVATE);
+
+    PFRU_IOC_STATGE: Stage a capsule image from communication buffer
+                    and perform authentication.
+    PFRU_IOC_ACTIVATE: Activate a previous staged capsule image.
+    PFRU_IOC_STAGE_ACTIVATE: Perform both stage and activation actions.
+
+PFRU Telemetry
+=============
+
+The user space tool manipulates on /dev/pfru/telemetry for PFRU telemetry log.
+Sample code:
+
+ 1. Open telemetry device
+    fd_log = open("/dev/pfru/telemetry", O_RDWR);
+
+ 2. Get log level, log type, revision_id via one ioctl invoke
+    ioctl(fd_log, PFRU_IOC_GET_LOG_INFO, &info);
+
+ 3. Set log level, log type, revision_id
+    ioctl(fd_log, PFRU_IOC_SET_LOG_INFO, &info);
+
+ 4. ioctl(fd_log, PFRU_IOC_GET_DATA_INFO, &data_info);
+    Query the information of PFRU telemetry log buffer. The user is
+    responsible for parsing the result per the specification.
+
+ 5. Read the telemetry data:
+    read(fd_log, buf, data_info.size);
+
+Please refer to tools/testing/selftests/pfru/pfru_test.c for detail.
+
+According to <Management Mode Firmware Runtime Update>[1], the telemetry
+buffer is a wrap around buffer. If the telemetry buffer gets full, most recent
+log data will overwrite old log data. Besides, it is required in the spec that
+the read of telemetry should support both full data retrieval and delta telemetry
+data retrieval. Since this requirement is more likely a policy we leave this
+implementation in user space. That is to say, it is recommended for the user
+to double-read the telemetry parameters such as chunk1_size, chunk2_size,
+rollover_cnt in data_info structure to make sure that there is no more data appended
+while the user is reading the buffer. Besides, only after the runtime update has
+been run at least once, the telemetry log would have valid data, otherwise errno code
+of EBUSY would be returned.
+
+[1] https://uefi.org/sites/default/files/resources/Intel_MM_OS_Interface_Spec_Rev100.pdf