From e8d8d9492da48430fe2c059fef8e9116fc17c188 Mon Sep 17 00:00:00 2001 From: Patrick Rudolph Date: Wed, 17 Apr 2019 11:51:25 +0200 Subject: 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 Reviewed-on: https://review.coreboot.org/c/coreboot/+/32340 Tested-by: build bot (Jenkins) Reviewed-by: Paul Menzel Reviewed-by: Hung-Te Lin Reviewed-by: Martin Roth --- Documentation/lib/timestamp.md | 176 +++++++++++++++++++++++++++++++++++++++++ 1 file changed, 176 insertions(+) create mode 100644 Documentation/lib/timestamp.md (limited to 'Documentation/lib/timestamp.md') diff --git a/Documentation/lib/timestamp.md b/Documentation/lib/timestamp.md new file mode 100644 index 0000000000..d5dc8facf3 --- /dev/null +++ b/Documentation/lib/timestamp.md @@ -0,0 +1,176 @@ +# Timestamps + +## 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: + +```c +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: + +```c +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: + +```c +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: + +```c +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. -- cgit v1.2.3