5 Power management (PM) is an event-driven state machine, tickled by various
6 `bta/sys` events via a callback. The actual state switching calls are handled
7 by the BTM HCI interfacing code, with results being posted back to the PM
8 code via the BTA workqueue thread.
10 Power states are managed per-device, per-profile, so every incoming event
11 includes a profile ID, app ID, and a `BD_ADDR`.
13 The events fired to drive the state machine at the time of this writing are:
16 - `BTA_SYS_CONN_CLOSE`
24 Each of these correspond to a function name in `bta/sys/bta_sys_conn.c`, which
25 are called by each profile definition in `bta/$PROFILE`.
27 The PM code makes calls into the BTM module to set various power
28 states. Responses are handled in an asynchronous fashion, primarily via the
29 callbacks `bta_dm_pm_cback` and `bta_dm_pm_timer_cback`. Responses are handled
30 through the BTA workqueue thread and the `bta_dm_pm_btm_status` function. Since
31 we might possibly get into a bad state where we never hear back from the
32 controller, timers are used to post messages to the BTA workqueue thread as
33 well, which filters down through the same status function.
35 Overall power states are managed *per device*, not per connection, but the power
36 policy is determined by the greatest allowable power action defined across all
37 currently known connections to a given device. Thus, if RFCOMM specifies that
38 it's willing to go to into SNIFF and specifies that as an action, and say, a PAN
39 connection is up which specifies it is willing to go into SNIFF, but its action
40 states it wants ACTIVE, the power management code will change to ACTIVE.
42 ### Power management tables
44 The tables that determine which power levels are acceptable for which profiles
45 and what actions to take for the above events are defined in the
46 `bta/dm/bta_dm_cfg.c` file, as `bta_dm_pm_cfg`, `bta_dm_pm_spec`, and
49 During a lookup attempt, the code iterates over the `bta_dm_pm_cfg` array,
50 looking for a match between the profile and app IDs. When it finds one, it uses
51 the `spec_idx` field to index into `bta_dm_pm_spec` array to determine which
52 power modes are acceptable and what actions to take for each event.
54 The action constants are defined in `bta_api.h` and are defined as a series of
55 hex bitfields. The actual actions taken are determined by the
56 `bta_dm_pm_set_mode` function, but a few of the actions listed deserve some
57 additional description:
59 - `BTA_DM_PM_NO_ACTION` is effectively a no-op and has a value of zero, so any
60 other profile will override this.
61 - `BTA_DM_PM_NO_PREF` overrides `BTA_DM_PM_NO_ACTION` and if selected as the
62 action that `bta_dm_pm_set_mode` will take, the connection will be removed
63 from `bta_dm_conn_srvcs` and no longer be considered for power management
65 - `BTA_DM_PM_SNIFF` through `BTA_DM_PM_SNIFF4` are special, in that each
66 level specifies a set of parameters for the SNIFF mode which relate to the
67 min and max intervals, the number of attempts and the timeout. The overall
68 action is still the same, however -- SNIFF mode is attempted. There are
69 definitions available up to SNIFF7, but actual SSR values are only defined
70 up to SNIFF4. Params are defined in `bta_dm_ssr_spec`.
71 - `BTA_DM_PM_ACTIVE` is full-on power.
72 - `BTA_DM_PM_RETRY` has the same effect as `BTA_DM_PM_NO_ACTION`, except a
73 timeout is possible to be set, which effectively allows a power operation to
78 `bta_dm_pm.c`'s `bta_dm_init_pm` function calls out to register
79 `bta_dm_pm_cback` with the bta sys module for incoming power management events,
80 and also registers `bta_dm_pm_btm_cback` with the btm module to handle responses
81 and timeouts of HCI requests (via `bta_dm_pm_btm_status`).
83 At this point, the power managment code is basically done until the first set of
84 events come in through `bta_dm_pm_cback`.
86 Throughout the `bta_dm_pm.c` file, connections whose power management states are
87 managed are tracked in a global array called `bta_dm_conn_srvcs`. Unfortunately,
88 while this variable is declared as an extern in the `bta_dm_int.h` file, it only
89 seems to be used in the `bta_dm_act.c` file, and only for reinitialization.
93 #### Events fired from SYS
95 1. An event is fired from one of the methods mentioned above in
96 `bta/sys/bta_sys_conn.c`
97 2. The `bta_dm_pm_cback` function is called.
98 - The power mode config is looked up in the `bta_dm_pm_cfg` table. If none
99 are found for the given profile ID and app ID, the function simply
100 returns with no action taken.
101 - If any timers were set for the given `BD_ADDR`, they are stopped.
102 - The SSR params for the CONN_OPEN event are looked up.
103 - The power spec state table (`bta_dm_pm_spec`) is checked to see if
104 there's no action to be performed (`BTA_DM_PM_NO_ACTION`), and if so,
105 returns with no action taken.
106 - `bta_dm_conn_srvcs` is consulted to ensure there's an entry for this
107 connection if it's supposed to be managed according to the power spec
108 state tables. If the spec specifies `BTA_DM_PM_NO_PREF`, then any
109 existing entry in this list is removed, otherwise one is added/updated
110 with the state given to the function.
111 3. `bta_dm_pm_cback` checks to see if the `bta_dm_ssr_spec` specifies SSR
112 adjustments are to be made, and if so, `bta_dm_pm_ssr` is called with the
114 - `bta_dm_pm_ssr` iterates the managed services array to find all connected
115 services for the given `BD_ADDR`, then looks up the ssr values from the
116 `bta_dm_ssr_spec` tables, looking for the smallest max latency to use.
117 - `bta_dm_pm_ssr` calls `BTM_SetSsrParams` to actually send along the SSR
118 params to the bluetooth chip.
119 4. `bta_dm_pm_cback` calls `bta_dm_pm_set_mode` with the peer address and the
120 `timed_out` parameter set to `false`.
121 - For each managed connection, `bta_dm_pm_set_mode` grabs
122 both actions specified for the profile in the `bta_dm_pm_spec` tables. If
123 the first power management action didn't timeout (or was never attempted,
124 according to the `tBTA_DM_PEER_DEVICE` `pm_mode_failed` and
125 `pm_mode_attempted` fields), its timeout and mode are used. Otherwise,
126 the same check is done against the second action and it is used
127 instead. If both actions have been attempted, then the action is set to
128 `BTA_DM_PM_NO_ACTION`. Only the highest power mode action is chosen from
129 all connected profiles.
130 - If the chosen action is `BTA_DM_PM_PARK` or `BTA_DM_PM_SNIFF` but the
131 profile doesn't allow it, this function takes no action.
132 - If a timeout is specified in the power spec table, then an unused timer
133 in `bta_dm_cb.pm_timer` is started.
134 - If the action chosen is `BTA_DM_PM_PARK`, `bta_dm_pm_park` is called,
135 which calls `BTM_ReadPowerMode` and `BTM_SetPowerMode` to make an HCI
136 request to enable PARK for the given peer and connection.
137 - If the action chosen is `BTA_DM_PM_SNIFF`, the peer device's link policy
138 is checked to see if it's allowed. If so, then `bta_dm_pm_sniff` is
139 called, which makes various calls to `BTM_ReadLocalFeatures`,
140 `BTM_ReadRemoteFeatures` and `BTM_SetPowerMode` to ensure SNIFF mode is
142 - If the action chosen is `BTA_DM_PM_ACTIVE`, a call to `bta_dm_pm_active`
143 is made, which calls `BTM_SetPowerMode` to set the link into ACTIVE
146 At this point, if one of the timers in `bta_dm_cb.pm_timer` times out, a call is
147 made through the BTA workqueue thread to `bta_dm_pm_btm_cback`, which then
148 triggers `bta_dm_pm_btm_status`, with the timeout field set to TRUE. HCI
149 responses are also fired as messages through the BTA workqueue thread, which are
150 handled again, through `bta_dm_pm_btm_status`.
152 #### Events fired through BTM
154 Essentially these messages eventually go through the same functions as events
155 fired from the SYS side of things, except from the initial path they take:
157 1. An event is fired from a callback in BTM to `bta_dm_pm_btm_cback`.
158 2. `bta_dm_pm_btm_cback` packages up the given parameters into a
159 `tBTA_DM_PM_BTM_STATUS` struct and posts it to the BTA workqueue thread via
160 `bta_sys_sendmsg`, with the event header set to
161 `BTA_DM_PM_BTM_STATUS_EVT`.
162 3. This is eventually routed to the `bta_dm_pm_btm_status` function.
163 **Determine if this is running on the workqueue thread or not**
164 - The message `status` passed in is actually the current status of the
166 - If the status is `BTM_PM_STS_ACTIVE` (still in the ACTIVE power mode),
167 checks the HCI status code:
168 - If that's non-zero and a PARK or SNIFF mode change was attempted,
169 `bta_dm_pm_btm_status` stops any timers started for the device in
170 `bta_dm_pm_set_mode`, clears some status bits in the peer device
171 structure, and then calls back into `bta_dm_pm_set_mode` with the peer
172 device address and timeout set to FALSE.
173 - If the status is zero, and if the peer device `tBTA_DM_PEER_DEVICE`
174 `prev_low` field is set, calls `bta_dm_pm_ssr` to re-send SSR params,
175 stops all timers for the device, and then re-calls `bta_dm_pm_set_mode`
176 with timeout set to FALSE to re-attempt with a second action (if the
177 previous PARK or SNIFF failed, otherwise it'll re-attempt the first
179 - If the status is `BTM_PM_STS_PARK` or `BTM_PM_STS_HOLD`, saves the
180 previous low power mode in the peer device's `prev_low` field.
181 - If the status is `BTM_PM_STS_SSR`, simply clears or sets the device
182 `info` field's `BTA_DM_DI_USE_SSR` bit, depending on the value of
183 `tBTA_DM_MSG.value`, which determines if the device can handle SSR.
184 - If the status is `BTM_PM_STS_SNIFF` and the info field has the
185 `BTA_DM_DI_SET_SNIFF` bit set, then `BTA_DM_DI_INT_SNIFF` is set,
186 otherwise `BTA_DM_DI_ACP_SNIFF` is set.
187 - If `BTA_PM_STS_ERROR`, the `BTA_DM_DI_SET_SNIFF` bit is cleared in the
190 At this point, either the method simply returns, or has called back into
191 `bta_dm_pm_set_mode`, in which case the usual flow takes over.
193 #### Events fired from timers
195 Timers are used exclusively for handling HCI command timeouts, and filter
196 through to a call to `bta_dm_pm_set_mode`:
198 1. A timer expires, and calls `bta_dm_pm_timer_cback`.
199 2. `bta_dm_pm_timer_cback` clears the use flag on the timer that fired, and
200 sends off an event to the BTA workqueue thread.
201 3. The event eventually fires off a call to `bta_dm_pm_timer`, which just
202 calls `bta_dm_pm_set_mode` with timeout set to `TRUE`.