﻿=========================== Table of Contents ==================================
Introduction

Data structures used
	cache_state
	cbmem_state
	table
	entries

Internal Functions
	timestamp_cache_init
	timestamp_cache_get
	timestamp_alloc_cbmem_table
	timestamp_table_get
	timestamp_add_table_entry

Function APIs
	timestamp_early_init
	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 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 initalize, 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 area under the CBMEM_ID_TIMESTAMP
tag. However, until the cbmem area is available, the timestamps can be saved to
board/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 _timestamp cache (if any used)
2. Initialize the base time and reset cbmem timestamp area
3. Start adding timestamps

	Steps 1) and 2) above can be performed in a single API call as explained
in later sections. 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 cbmem timestamp
region is required / not.

========================== Data structures used ================================

	The main structure that maintains information about the timestamp cache
is:
struct __attribute__((__packed__)) timestamp_cache {
        uint16_t cache_state;
        uint16_t cbmem_state;
        struct timestamp_table table;
        struct timestamp_entry entries[MAX_TIMESTAMP_CACHE];
};

1. 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
_timestamp region is being used by any board, it is important to call
timestamp_early_init which provides the initialization of timestamp cache.

	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.

	When cbmem area is up (i.e. call to cbmem_initialize), 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 cbmem area.


2. cbmem_state
	This attribute indicates if we need to reset the timestamp area in
cbmem. This needs to be done for fresh boot (when there is no timestamp area
allocated in cbmem) or during resume (to clean out timestamps from earlier
boot). This can be set to any one of the following values:

enum {
        TIMESTAMP_CBMEM_RESET_NOT_REQD = 0,
        TIMESTAMP_CBMEM_RESET_REQD,
};

	If _timestamp region is being used, the call to timestamp_early_init
sets this field to reset_reqd. If this region is not being used, then instead
the call to timestamp_init sets this field to reset_reqd. Thus, when cbmem
initialization is complete, the timestamp library ensures that the cbmem
timestamp area is properly set as per the value of this attribute and resets
this field to reset_not_reqd. In case of resume path, this field would be set
accordingly by timestamp_early_init  / timestamp_init thus ensuring that
timestamps from earlier boot dont float around into current boot logs. Also,
during early ramstage, when cbmem is not yet up, local stash would be used which
is in the bss area and thus this field is set to reset_not_reqd (unless
timestamp_init is called before or after cbmem_intiialize). Hence, when ramstage
brings up cbmem and syncs timestamps, the cbmem timestamp area is not reset. If
ramstage is the first stage where timestamp_init is called, then the cbmem area
is accordingly reset in timestamp_sync  or timestamp_init based on the order in
which calls are made. Details are explained in the use cases towards the end.


3. 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.


4. 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.


========================== Internal Functions ==================================

1. timestamp_cache_init
	Initializes the timestamp cache to a known state by setting num_entries
to 0, base_entries to MAX_TIMESTAMP_CACHE, base_time to caller-provided base,
cache_state to TIMESTAMP_CACHE_INITIALIZED and cbmem_state to caller-provided
state.


2.  timestamp_cache_get
	This function returns a pointer to timestamp cache. It basically
identifies what cache to use based on following parameters:
a. If __PRE_RAM__ is true i.e. we are in pre-ramstage step and _timestamp region
is provided, use _timestamp region.
b. Otherwise use local stash

	If using _timestamp region, it ensures that the size of the region is
big enough to hold all of timestamp_cache structure and MAX_TIMESTAMP_CACHE
timestamp entries. Once the location of cache is decided, it checks to see if
cache is not yet initialized, and calls timestamp_cache_init if required.


3. timestamp_alloc_cbmem_table
	This function exists only in romstage and ramstage since only these two
stages access cbmem area. It allocates a new table for MAX_TIMESTAMPS number of
entries in the cbmem area using CBMEM_ID_TIMESTAMP tag. It also resets the
base_time and num_entries to 0.


4. timestamp_table_get
	This function returns a pointer to the timestamp table. Based on the
value of cache_state in timestamp_cache, it provides a pointer either to the
cache (in stash or _timestamp region) or the cbmem timestamp table. If a pointer
to the cbmem timestamp table is returned, it checks to see if cbmem_state
indicates a reset of the cbmem timestamp area. If required, it makes a call to
timestamp_alloc_cbmem_table.


5. timestamp_add_table_entry
	This function adds a new entry to the timestamp table obtained by making
a call to timestamp_table_get. It sets the id based on user-provided id and the
stamp is set to user-provided time - table base time. This is to normalize all
the timestamps.


============================= Function APIs ====================================

1. timestamp_early_init
	It is essential to call this function only if _timestamp region is being
used. This should be the first call made to the timestamp library. Since
_timestamp region can contain arbitary values in the different fields of the
cache, this function initializes the fields properly by calling
timestamp_cache_init with the cache pointer, user-provided base time and
cbmem_state as TIMESTAMP_CBMEM_RESET_REQD. This will provide an indication that
cbmem timestamp area needs to be added (in case of fresh boot) or reset to 0
entries(in case of resume).


2. timestamp_init
	It is essential to call this function before any call to timestamp_add
is made. In case of _timestamp region being used, only a call to
timestamp_early_init is required. In all other cases, it is essential to call
timestamp_init to set the cbmem_state to TIMESTAMP_CBMEM_RESET_REQD. This will
provide an indication that cbmem timestamp area needs to be added (in case of
fresh boot) or reset to 0 entries(in case of resume).


3. timestamp_add
	This function accepts from user a timestamp id and time to record in the
timestamp table. Based on the table provided by timestamp_table_get, it stores
the entry in the appropriate table in cbmem or _timestamp region or local
stash.


4. timestamp_add_now
	This function calls timestamp_add with user-provided id and current
time.


5. timestamp_sync
	This is one of the most important functions required to ensure the
proper functioning of the timestamp library across cbmem, _timestamp region and
local stash with minimal interference for the user. It is important to note that
this function is called only by cbmem_initialize to ensure that once the cbmem
area is ready, we automatically sync all the timestamps saved in the cache to
cbmem area. Thus, the user never has to call the timestamp_sync function.

	This function checks to see if the cache state indicates that a cbmem
reset of timestamp area is required or no timestamp area exists in cbmem at
all. In that case, it allocates a new cbmem area by calling
timestamp_alloc_cbmem_table.

	Otherwise, it uses cbmem_find to use the timestamp area in cbmem. It
then uses timestamp_add_table_entry to move all the entries from cache to cbmem
area. Here, two cases can occur with respect to the base time subtraction:

a. Newly added cbmem table will have a base time of 0. Thus, no adjustments are
required for the timestamps being added from cache to cbmem table.

b. Timestamps added to cache before ramstage: In this case, the base time in
cache table will be 0 and timestamp_add_table_entry will take care of
subtracting the correct base_time. Finally, it resets the timestamp cache to:

        cache_state: TIMESTAMP_CACHE_NOT_NEEDED
        cbmem_state: TIMESTAMP_CBMEM_RESET_NOT_REQD
        num_entries: 0

	Also, the base_time in cbmem table is update if it is currently zero
indicating that it needs to inherit base time from cache table since
cbmem table might be newly allocated.


============================= 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)


1. timestamp_early_init is called to setup the cache state and cbmem_state is
set to reset_reqd. These properties are set in the timestamp cache which is part
of _timestamp region.

2. <Any number of timestamp_add / timestamp_add_now> : All saved in _timestamp
region

3. Once cbmem_initiliaze is complete, it calls timestamp_sync, which checks the
_timestamp region to check the cbmem_state field. Since it says reset_reqd, a
new area for timestamp is setup in cbmem by calling cbmem_add. The value of
cbmem_state is set to reset_not_reqd in _timestamp region cache.

4. After entering ramstage, the local stash acts as the timestamp cache. Since
it is in BSS area, the cache_state field is set to uninitialized and cbmem_state
field is set to reset_not_reqd.

5. On any call to timestamp_add / timestamp_add_now, it initializes the cache
and sets cache_state field to initialized whereas cbmem_state is still set to
reset_not_reqd.

6. Once cbmem_initiliaze is complete, it calls timestamp_sync, which checks the
local stash to check the cbmem_state field. Since it says reset_not_reqd, it
uses cbmem_find to find already allocated timestamp area in cbmem. If none is
found, it allocates a new one.

7. <Any number of calls to timestamp_add / timestamp_add_now> : All saved in
cbmem


Case 2: No timestamp region, fresh boot, cbmem_initialize called after
timestamp_init

1. Since timestamp_init is called before cbmem_initialize, it will set the cache
field attributes to: cache_state as initialized and cbmem_state as
reset_reqd. This ensures that timestamp_sync sets up a new cbmem area for
timestamps.

2. < Any number of calls to timestamp_add / timestamp_add_now> : All saved in
stash

3. Once cbmem_initiliaze is complete, it calls timestamp_sync, which checks the
stash cache to check the cbmem_state field. Since it says reset_reqd, a new area
for timestamp is setup in cbmem by calling cbmem_add. The value of cbmem_state
is set to reset_not_reqd in stash cache.

4. After entering ramstage, a new local stash acts as the timestamp cache. Since
it is in BSS area, the cache_state field is set to uninitialized and cbmem_state
field is set to reset_not_reqd.

5. On any call to timestamp_add / timestamp_add_now, it initializes the cache
and sets cache_state field to initialized whereas cbmem_state is still set to
reset_not_reqd.

6. Once cbmem_initiliaze is complete, it calls timestamp_sync, which checks the
local stash to check the cbmem_state field. Since it says reset_not_reqd, it
uses cbmem_find to find already allocated timestamp area in cbmem. If none is
found, it allocates a new one.

7. <Any number of calls to timestamp_add / timestamp_add_now> : All saved in
cbmem


Case 3: No timestamp region, fresh boot, cbmem_initialize called before
timestamp_init

1. Since cbmem_initialize is called before timestamp_init, it will call
timestamp_sync which checks local stash to check for cbmem_state field. Since,
cbmem_state says no_reset_reqd, it uses cbmem_find to look for a timestamp area
in cbmem. Since, it is a fresh boot, none will be found and it uses cbmem_add to
create a new area on cbmem.

2. timestamp_init is called, which sets local cache state to reset required. It
then calls timestamp_table_get which sees that cache_state is cache_not_needed
and cbmem_state is reset_reqd. Thus, it calls timestamp_alloc_cbmem_table to
reset the cbmem area which is as good as no change in the state of cbmem
area. Then, the cbmem_state is set to reset_not_reqd.

3. <Any number of calls to timestamp_add / timestamp_add_now> : All saved in
cbmem

4. After entering ramstage, a new local stash acts as the timestamp cache. Since
it is in BSS area, the cache_state field is set to uninitialized and cbmem_state
field is set to reset_not_reqd.

5. On any call to timestamp_add / timestamp_add_now, it initializes the cache
and sets cache_state field to initialized whereas cbmem_state is still set to
reset_not_reqd.

6. Once cbmem_initiliaze is complete, it calls timestamp_sync, which checks the
local stash to check the cbmem_state field. Since it says reset_not_reqd, it
uses cbmem_find to find already allocated timestamp area in cbmem. If none is
found, it allocates a new one.

7. <Any number of calls to timestamp_add / timestamp_add_now> : All saved in
cbmem


Case 4: No timestamp region, resume, cbmem_initialize called after
timestamp_init

1. Since timestamp_init is called before cbmem_initialize, it will set the cache
field attributes to: cache_state as initialized and cbmem_state as reset_reqd.

2. < Any number of calls to timestamp_add / timestamp_add_now> : All saved in
stash

3. Once cbmem_initiliaze is complete, it calls timestamp_sync, which checks the
stash cache to check the cbmem_state field. Since it says reset_reqd,
timestamp_sync resets cbmem area to num_entries=0, thus flushing timestamps from
earlier boot. The value of cbmem_state is set to reset_not_reqd in stash cache.

4. After entering ramstage, a new local stash acts as the timestamp cache. Since
it is in BSS area, the cache_state field is set to uninitialized and cbmem_state
field is set to reset_not_reqd.

5. On any call to timestamp_add / timestamp_add_now, it initializes the cache
and sets cache_state field to initialized whereas cbmem_state is still set to
reset_not_reqd.

6. Once cbmem_initiliaze is complete, it calls timestamp_sync, which checks the
local stash to check the cbmem_state field. Since it says reset_not_reqd, it
uses cbmem_find to find already allocated timestamp area in cbmem. If none is
found, it allocates a new one.

7. <Any number of calls to timestamp_add / timestamp_add_now> : All saved in
cbmem


Case 5: No timestamp region, resume, cbmem_initialize called before
timestamp_init

1. Since cbmem_initialize is called before timestamp_init, it will call
timestamp_sync which checks local stash to check for cbmem_state field. Since,
cbmem_state says no_reset_reqd, it uses cbmem_find to look for a timestamp area
in cbmem. Since, it is resume, it will find the older cbmem timestamp area from
earlier boot. However, nothing is added to cbmem area since timestamp_add /
timestamp_add_now is expected to be used only after call to timestamp_init.

2. timestamp_init is called, which sets local cache state to reset required. It
then calls timestamp_table_get which sees that cache_state is cache_not_needed
and cbmem_state is reset_reqd. Thus, it calls timestamp_alloc_cbmem_table to
reset the cbmem area which sets num_entries to 0. Thus, flushing timestamps from
earlier boot. Then, the cbmem_state is set to reset_not_reqd.

3. <Any number of calls to timestamp_add / timestamp_add_now> : All saved in
cbmem

4. After entering ramstage, a new local stash acts as the timestamp cache. Since
it is in BSS area, the cache_state field is set to uninitialized and cbmem_state
field is set to reset_not_reqd.

5. On any call to timestamp_add / timestamp_add_now, it initializes the cache
and sets cache_state field to initialized whereas cbmem_state is still set to
reset_not_reqd.

6. Once cbmem_initiliaze is complete, it calls timestamp_sync, which checks the
local stash to check the cbmem_state field. Since it says reset_not_reqd, it
uses cbmem_find to find already allocated timestamp area in cbmem. If none is
found, it allocates a new one.

7. <Any number of calls to timestamp_add / timestamp_add_now> : All saved in
cbmem