documentation: Add documentation for timestamp library

[pg: removed discussion of timestamp internals that isn't current anymore in
favor of some notes for users: when to run which function, what _not_ to do.
Also moved to markdown-ish layout. Will do further style cleanups later.]

BUG=chrome-os-partner:32973
BRANCH=None
TEST=None

Change-Id: I6ea7237f2fa749ce3a493f378f9937e642f3b678
Signed-off-by: Marc Jones <marc.jones@se-eng.com>
Signed-off-by: Patrick Georgi <pgeorgi@chromium.org>
Original-Commit-Id: 97e2a3ebd9552c2a91d9ea62be515059428631cb
Original-Change-Id: I4b184ffad6fcd93d63343a9bca34ad013e9d4263
Original-Signed-off-by: Furquan Shaikh <furquan@google.com>
Original-Reviewed-on: https://chromium-review.googlesource.com/229861
Original-Tested-by: Furquan Shaikh <furquan@chromium.org>
Original-Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Original-Commit-Queue: Furquan Shaikh <furquan@chromium.org>
Reviewed-on: http://review.coreboot.org/10741
Tested-by: build bot (Jenkins)
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>

1 files changed, 200 insertions, 0 deletions

diff --git a/Documentation/timestamp.md b/Documentation/timestamp.md
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+Table of Contents
+=================
+Introduction
+	Transition from cache to cbmem
+
+Data structures used
+	cache_state
+	table
+	entries
+
+Function APIs
+	timestamp_init
+	timestamp_add
+	timestamp_add_now
+	timestamp_sync
+
+Use / Test Cases
+	Case 1: Timestamp Region Exists
+	Case 2: No timestamp region, fresh boot, cbmem_initialize called after
+	timestamp_init
+	Case 3: No timestamp region, fresh boot, cbmem_initialize called before
+	timestamp_init
+	Case 4: No timestamp region, resume, cbmem_initialize called after
+	timestamp_init
+	Case 5: No timestamp region, resume, cbmem_initialize called before
+	timestamp_init
+
+
+Introduction
+============
+The aim of the timestamp library is to make it easier for different boards
+to  save timestamps in cbmem / stash (until cbmem is brought up) by
+providing a simple API to initialize, add and sync timestamps. In order
+to make the timestamps persistent and accessible from the kernel, we
+need to ensure that all the saved timestamps end up in cbmem under
+the CBMEM_ID_TIMESTAMP tag. However, until the cbmem area is available,
+the timestamps can be saved to a SoC-defined \_timestamp region or in a
+local stage-specific stash. The work of identifying the right location for
+storing timestamps is done by the library and is not exposed to the user.
+
+Working of timestamp library from a user perspective can be outlined in
+the following steps:
+1. Initialize the base time and reset cbmem timestamp area
+2. Start adding timestamps
+
+Behind the scenes, the timestamp library takes care of:
+1. Identifying the correct location for storing timestamps (cbmem or timestamp
+   region or local stash).
+2. Once cbmem is up, ensure that all timestamps are synced from timestamp
+   region or local stash into the cbmem area.
+3. Add a new cbmem timestamp area based on whether a reset of the cbmem
+   timestamp region is required or not.
+
+Transition from cache to cbmem
+------------------------------
+To move timestamps from the cache to cbmem (and initialize the cbmem area in
+the first place), we use the CBMEM_INIT_HOOK infrastructure of coreboot.
+
+When cbmem is initialized, the hook is called, which creates the area,
+copies all timestamps to cbmem and disables the cache.
+
+After such a transition, timestamp_init() must not be run again.
+
+
+Data structures used
+====================
+The main structure that maintains information about the timestamp cache is:
+struct __attribute__((__packed__)) timestamp_cache {
+        uint16_t cache_state;
+        struct timestamp_table table;
+        struct timestamp_entry entries[MAX_TIMESTAMP_CACHE];
+};
+
+cache_state
+-----------
+The state of the cache is maintained by cache_state attribute which can
+be any one of the following:
+
+enum {
+        TIMESTAMP_CACHE_UNINITIALIZED = 0,
+        TIMESTAMP_CACHE_INITIALIZED,
+        TIMESTAMP_CACHE_NOT_NEEDED,
+};
+
+By default, if the cache is stored in local stash (bss area), then
+it will be reset to uninitialized state. However, if the cache is
+stored in timestamp region, then it might have garbage in any of the
+attributes. Thus, if the timestamp region is being used by any board, it is
+initialized to default values by the library.
+
+Once the cache is initialized, its state is set to
+CACHE_INITIALIZED. Henceforth, the calls to cache i.e. timestamp_add
+know that the state reflected is valid and timestamps can be directly
+saved in the cache.
+
+Once the cbmem area is up (i.e. call to timestamp_sync_cache_to_cbmem),
+we do not need to store the timestamps in local stash / timestamp area
+anymore. Thus, the cache state is set to CACHE_NOT_NEEDED, which allows
+timestamp_add to store all timestamps directly into the cbmem area.
+
+
+table
+-----
+This field is represented by a structure which provides overall
+information about the entries in the timestamp area:
+
+struct timestamp_table {
+        uint64_t        base_time;
+        uint32_t        max_entries;
+        uint32_t        num_entries;
+        struct timestamp_entry entries[0]; /* Variable number of entries */
+} __attribute__((packed));
+
+It indicates the base time for all timestamp entries, maximum number
+of entries that can be stored, total number of entries that currently
+exist and an entry structure to hold variable number of entries.
+
+
+entries
+-------
+This field holds the details of each timestamp entry, upto a maximum
+of MAX_TIMESTAMP_CACHE which is defined as 16 entries. Each entry is
+defined by:
+
+struct timestamp_entry {
+        uint32_t        entry_id;
+        uint64_t        entry_stamp;
+} __attribute__((packed));
+
+entry_id holds the timestamp id corresponding to this entry and
+entry_stamp holds the actual timestamp.
+
+
+For timestamps stored in the cbmem area, a timestamp_table is allocated
+with space for MAX_TIMESTAMPS equal to 30. Thus, the cbmem area holds
+base_time, max_entries (which is 30), current number of entries and the
+actual entries represented by timestamp_entry.
+
+
+Function APIs
+=============
+
+timestamp_init
+--------------
+This function initializes the timestamp cache and should be run as early
+as possible. On platforms with SRAM, this might mean in bootblock, on
+x86 with its CAR backed memory in romstage, this means romstage before
+memory init.
+
+timestamp_add
+-------------
+This function accepts from user a timestamp id and time to record in the
+timestamp table. It stores the entry in the appropriate table in cbmem
+or _timestamp region or local stash.
+
+
+timestamp_add_now
+-----------------
+This function calls timestamp_add with user-provided id and current time.
+
+
+Use / Test Cases
+================
+
+The following cases have been considered while designing the timestamp
+library. It is important to ensure that any changes made to this library satisfy
+each of the following use cases:
+
+Case 1: Timestamp Region Exists (Fresh Boot / Resume)
+-----------------------------------------------------
+
+In this case, the library needs to call timestamp_init as early as possible to
+enable the timestamp cache. Once cbmem is available, the values will be
+transferred automatically.
+
+All regions are automatically reset on initialization.
+
+Case 2: No timestamp region, fresh boot, cbmem_initialize called after timestamp_init
+-------------------------------------------------------------------------------------
+
+timestamp_init will set up a local cache. cbmem must be initialized before that
+cache vanishes - as happens when jumping to the next stage.
+
+Case 3: No timestamp region, fresh boot, cbmem_initialize called before timestamp_init
+--------------------------------------------------------------------------------------
+
+This case is not supported right now, just don't call timestamp_init after
+cbmem_initialize. (Patches to make this more robust are welcome.)
+
+Case 4: No timestamp region, resume, cbmem_initialize called after timestamp_init
+---------------------------------------------------------------------------------
+
+We always reset the cbmem region before using it, so pre-suspend timestamps
+will be gone.
+
+Case 5: No timestamp region, resume, cbmem_initialize called before timestamp_init
+----------------------------------------------------------------------------------
+
+We always reset the cbmem region before using it, so pre-suspend timestamps
+will be gone.