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android13/packages/modules/Bluetooth/system/doc/power_management.md

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Power Management

Overview

Power management (PM) is an event-driven state machine, tickled by various bta/sys events via a callback. The actual state switching calls are handled by the BTM HCI interfacing code, with results being posted back to the PM code via the BTA workqueue thread.

Power states are managed per-device, per-profile, so every incoming event includes a profile ID, app ID, and a BD_ADDR.

The events fired to drive the state machine at the time of this writing are:

  • BTA_SYS_CONN_OPEN
  • BTA_SYS_CONN_CLOSE
  • BTA_SYS_CONN_IDLE
  • BTA_SYS_CONN_BUSY
  • BTA_SYS_APP_OPEN
  • BTA_SYS_APP_CLOSE
  • BTA_SYS_SCO_OPEN
  • BTA_SYS_SCO_CLOSE

Each of these correspond to a function name in bta/sys/bta_sys_conn.cc, which are called by each profile definition in bta/$PROFILE.

The PM code makes calls into the BTM module to set various power states. Responses are handled in an asynchronous fashion, primarily via the callbacks bta_dm_pm_cback and bta_dm_pm_timer_cback. Responses are handled through the BTA workqueue thread and the bta_dm_pm_btm_status function. Since we might possibly get into a bad state where we never hear back from the controller, timers are used to post messages to the BTA workqueue thread as well, which filters down through the same status function.

Overall power states are managed per device, not per connection, but the power policy is determined by the greatest allowable power action defined across all currently known connections to a given device. Thus, if RFCOMM specifies that it's willing to go to into SNIFF and specifies that as an action, and say, a PAN connection is up which specifies it is willing to go into SNIFF, but its action states it wants ACTIVE, the power management code will change to ACTIVE.

Power management tables

The tables that determine which power levels are acceptable for which profiles and what actions to take for the above events are defined in the bta/dm/bta_dm_cfg.cc file, as bta_dm_pm_cfg, bta_dm_pm_spec, and bta_dm_ssr_spec.

During a lookup attempt, the code iterates over the bta_dm_pm_cfg array, looking for a match between the profile and app IDs. When it finds one, it uses the spec_idx field to index into bta_dm_pm_spec array to determine which power modes are acceptable and what actions to take for each event.

The action constants are defined in bta_api.h and are defined as a series of hex bitfields. The actual actions taken are determined by the bta_dm_pm_set_mode function, but a few of the actions listed deserve some additional description:

  • BTA_DM_PM_NO_ACTION is effectively a no-op and has a value of zero, so any other profile will override this.
  • BTA_DM_PM_NO_PREF overrides BTA_DM_PM_NO_ACTION and if selected as the action that bta_dm_pm_set_mode will take, the connection will be removed from bta_dm_conn_srvcs and no longer be considered for power management decisions.
  • BTA_DM_PM_SNIFF through BTA_DM_PM_SNIFF6 are special, in that each level specifies a set of parameters for the SNIFF mode which relate to the min and max intervals, the number of attempts and the timeout. The overall action is still the same, however -- SNIFF mode is attempted. There are definitions available up to SNIFF7, but actual SSR values are only defined up to SNIFF6. Params are defined in bta_dm_ssr_spec.
  • BTA_DM_PM_ACTIVE is full-on power.
  • BTA_DM_PM_RETRY has the same effect as BTA_DM_PM_NO_ACTION, except a timeout is possible to be set, which effectively allows a power operation to be "retried".

Initialization

bta_dm_pm.cc's bta_dm_init_pm function calls out to register bta_dm_pm_cback with the bta sys module for incoming power management events, and also registers bta_dm_pm_btm_cback with the btm module to handle responses and timeouts of HCI requests (via bta_dm_pm_btm_status).

At this point, the power managment code is basically done until the first set of events come in through bta_dm_pm_cback.

Throughout the bta_dm_pm.cc file, connections whose power management states are managed are tracked in a global array called bta_dm_conn_srvcs. Unfortunately, while this variable is declared as an extern in the bta_dm_int.h file, it only seems to be used in the bta_dm_act.cc file, and only for reinitialization.

Event flow

Events fired from SYS

  1. An event is fired from one of the methods mentioned above in bta/sys/bta_sys_conn.cc
  2. The bta_dm_pm_cback function is called.
    • The power mode config is looked up in the bta_dm_pm_cfg table. If none are found for the given profile ID and app ID, the function simply returns with no action taken.
    • If any timers were set for the given BD_ADDR, they are stopped.
    • The SSR params for the CONN_OPEN event are looked up.
    • The power spec state table (bta_dm_pm_spec) is checked to see if there's no action to be performed (BTA_DM_PM_NO_ACTION), and if so, returns with no action taken.
    • bta_dm_conn_srvcs is consulted to ensure there's an entry for this connection if it's supposed to be managed according to the power spec state tables. If the spec specifies BTA_DM_PM_NO_PREF, then any existing entry in this list is removed, otherwise one is added/updated with the state given to the function.
  3. bta_dm_pm_cback checks to see if the bta_dm_ssr_spec specifies SSR adjustments are to be made, and if so, bta_dm_pm_ssr is called with the peer BD_ADDR.
    • bta_dm_pm_ssr iterates the managed services array to find all connected services for the given BD_ADDR, then looks up the ssr values from the bta_dm_ssr_spec tables, looking for the smallest max latency to use.
    • bta_dm_pm_ssr calls BTM_SetSsrParams to actually send along the SSR params to the bluetooth chip.
  4. bta_dm_pm_cback calls bta_dm_pm_set_mode with the peer address and the timed_out parameter set to false.
    • For each managed connection, bta_dm_pm_set_mode grabs both actions specified for the profile in the bta_dm_pm_spec tables. If the first power management action didn't timeout (or was never attempted, according to the tBTA_DM_PEER_DEVICE pm_mode_failed and pm_mode_attempted fields), its timeout and mode are used. Otherwise, the same check is done against the second action and it is used instead. If both actions have been attempted, then the action is set to BTA_DM_PM_NO_ACTION. Only the highest power mode action is chosen from all connected profiles.
    • If the chosen action is BTA_DM_PM_PARK or BTA_DM_PM_SNIFF but the profile doesn't allow it, this function takes no action.
    • If a timeout is specified in the power spec table, then an unused timer in bta_dm_cb.pm_timer is started.
    • If the action chosen is BTA_DM_PM_PARK, bta_dm_pm_park is called, which calls BTM_ReadPowerMode and BTM_SetPowerMode to make an HCI request to enable PARK for the given peer and connection.
    • If the action chosen is BTA_DM_PM_SNIFF, the peer device's link policy is checked to see if it's allowed. If so, then bta_dm_pm_sniff is called, which makes various calls to the local controller, BTM_ReadRemoteFeatures and BTM_SetPowerMode to ensure SNIFF mode is enabled.
    • If the action chosen is BTA_DM_PM_ACTIVE, a call to bta_dm_pm_active is made, which calls BTM_SetPowerMode to set the link into ACTIVE mode.

At this point, if one of the timers in bta_dm_cb.pm_timer times out, a call is made through the BTA workqueue thread to bta_dm_pm_btm_cback, which then triggers bta_dm_pm_btm_status, with the timeout field set to TRUE. HCI responses are also fired as messages through the BTA workqueue thread, which are handled again, through bta_dm_pm_btm_status.

Events fired through BTM

Essentially these messages eventually go through the same functions as events fired from the SYS side of things, except from the initial path they take:

  1. An event is fired from a callback in BTM to bta_dm_pm_btm_cback.
  2. bta_dm_pm_btm_cback packages up the given parameters into a tBTA_DM_PM_BTM_STATUS struct and posts it to the BTA workqueue thread via bta_sys_sendmsg, with the event header set to BTA_DM_PM_BTM_STATUS_EVT.
  3. This is eventually routed to the bta_dm_pm_btm_status function. Determine if this is running on the workqueue thread or not
    • The message status passed in is actually the current status of the device.
    • If the status is BTM_PM_STS_ACTIVE (still in the ACTIVE power mode), checks the HCI status code:
      • If that's non-zero and a PARK or SNIFF mode change was attempted, bta_dm_pm_btm_status stops any timers started for the device in bta_dm_pm_set_mode, clears some status bits in the peer device structure, and then calls back into bta_dm_pm_set_mode with the peer device address and timeout set to FALSE.
      • If the status is zero, and if the peer device tBTA_DM_PEER_DEVICE prev_low field is set, calls bta_dm_pm_ssr to re-send SSR params, stops all timers for the device, and then re-calls bta_dm_pm_set_mode with timeout set to FALSE to re-attempt with a second action (if the previous PARK or SNIFF failed, otherwise it'll re-attempt the first action).
    • If the status is BTM_PM_STS_PARK or BTM_PM_STS_HOLD, saves the previous low power mode in the peer device's prev_low field.
    • If the status is BTM_PM_STS_SSR, simply clears or sets the device info field's BTA_DM_DI_USE_SSR bit, depending on the value of tBTA_DM_MSG.value, which determines if the device can handle SSR.
    • If the status is BTM_PM_STS_SNIFF and the info field has the BTA_DM_DI_SET_SNIFF bit set, then BTA_DM_DI_INT_SNIFF is set, otherwise BTA_DM_DI_ACP_SNIFF is set.
    • If BTA_PM_STS_ERROR, the BTA_DM_DI_SET_SNIFF bit is cleared in the device info field.

At this point, either the method simply returns, or has called back into bta_dm_pm_set_mode, in which case the usual flow takes over.

Events fired from timers

Timers are used exclusively for handling HCI command timeouts, and filter through to a call to bta_dm_pm_set_mode:

  1. A timer expires, and calls bta_dm_pm_timer_cback.
  2. bta_dm_pm_timer_cback clears the use flag on the timer that fired, and sends off an event to the BTA workqueue thread.
  3. The event eventually fires off a call to bta_dm_pm_timer, which just calls bta_dm_pm_set_mode with timeout set to TRUE.