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author | Patrick Rudolph <patrick.rudolph@9elements.com> | 2019-04-17 11:51:25 +0200 |
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committer | Patrick Rudolph <siro@das-labor.org> | 2019-04-19 11:36:53 +0000 |
commit | e8d8d9492da48430fe2c059fef8e9116fc17c188 (patch) | |
tree | 5cea980621185dd546a45c0a6be29819fc6d82f4 /Documentation/timestamp.md | |
parent | 8f702676071ab3e62a9c07d5bdc75f5c92e58946 (diff) | |
download | coreboot-e8d8d9492da48430fe2c059fef8e9116fc17c188.tar.xz |
Documentation: Add small fixes
* Remove empty security.md
* Remove second H1 header from lib/index.md
* Move two documents in appropriate subfolders
* Fix file path
* Drop document overview
Change-Id: I0e9df6203e82003c01b84967ea6bd779d7583fef
Signed-off-by: Patrick Rudolph <patrick.rudolph@9elements.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/32340
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Hung-Te Lin <hungte@chromium.org>
Reviewed-by: Martin Roth <martinroth@google.com>
Diffstat (limited to 'Documentation/timestamp.md')
-rw-r--r-- | Documentation/timestamp.md | 200 |
1 files changed, 0 insertions, 200 deletions
diff --git a/Documentation/timestamp.md b/Documentation/timestamp.md deleted file mode 100644 index 9233ed97cc..0000000000 --- a/Documentation/timestamp.md +++ /dev/null @@ -1,200 +0,0 @@ -# Timestamps - -## 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 __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 */ -} __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; -} __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. |