health: add README.

Upgrading from health@1.0 HAL

0. Remove android.hardware.health@1.0* from PRDOUCT_PACKAGES

   in device/<manufacturer>/<device>/device.mk

1. If the device does not have a vendor-specific libhealthd AND does not

   implement storage-related APIs, just add the following to PRODUCT_PACKAGES:

   android.hardware.health@2.0-service

   Otherwise, continue to Step 2.

2. Create directory

   device/<manufacturer>/<device>/health

3. Create device/<manufacturer>/<device>/health/Android.bp

   (or equivalent device/<manufacturer>/<device>/health/Android.mk)

cc_binary {

    name: "android.hardware.health@2.0-service.<device>",

    init_rc: ["android.hardware.health@2.0-service.<device>.rc"],

    proprietary: true,

    relative_install_path: "hw",

    srcs: [

        "HealthService.cpp",

    ],

    cflags: [

        "-Wall",

        "-Werror",

    ],

    static_libs: [

        "android.hardware.health@2.0-impl",

        "android.hardware.health@1.0-convert",

        "libhealthservice",

        "libbatterymonitor",

    ],

    shared_libs: [

        "libbase",

        "libcutils",

        "libhidlbase",

        "libhidltransport",

        "libutils",

        "android.hardware.health@2.0",

    ],

    header_libs: ["libhealthd_headers"],

}

4. Create device/<manufacturer>/<device>/health/android.hardware.health@2.0-service.<device>.rc

service vendor.health-hal-2-0 /vendor/bin/hw/android.hardware.health@2.0-service.<device>

    class hal

    user system

    group system

5. Create device/<manufacturer>/<device>/health/HealthService.cpp:

#include <health2/service.h>

int main() { return health_service_main(); }

6. libhealthd dependency:

6.1 If the device has a vendor-specific libhealthd.<soc>, add it to static_libs.

6.2 If the device does not have a vendor-specific libhealthd, add the following

    lines to HealthService.cpp:

#include <healthd/healthd.h>

void healthd_board_init(struct healthd_config*) {}

int healthd_board_battery_update(struct android::BatteryProperties*) {

    // return 0 to log periodic polled battery status to kernel log

    return 0;

}

7. Storage related APIs:

7.1 If the device does not implement IHealth.getDiskStats and

    IHealth.getStorageInfo, add libstoragehealthdefault to static_libs.

7.2 If the device implements one of these two APIs, add and implement the

    following functions in HealthService.cpp:

void get_storage_info(std::vector<struct StorageInfo>& info) {

    // ...

}

void get_disk_stats(std::vector<struct DiskStats>& stats) {

    // ...

}

8. Update necessary SELinux permissions. For example,

# device/<manufacturer>/<device>/sepolicy/vendor/file_contexts

/vendor/bin/hw/android\.hardware\.health@2\.0-service.<device> u:object_r:hal_health_default_exec:s0

# device/<manufacturer>/<device>/sepolicy/vendor/hal_health_default.te

# Add device specific permissions to hal_health_default domain, especially

# if Step 6.2 or Step 7.2 is done.

* libhealthhalutils

A convenience library for (hwbinder) clients of health HAL to choose between

the "default" instance (served by vendor service) or "backup" instance (served

by healthd). C++ clients of health HAL should use this library instead of

calling IHealth::getService() directly.

Its Java equivalent can be found in BatteryService.HealthServiceWrapper.

* libhealthservice

Common code for all (hwbinder) services of the health HAL, including healthd and

vendor health service android.hardware.health@2.0-service(.<vendor>). main() in

those binaries calls health_service_main() directly.

* libhealthstoragedefault

Default implementation for storage related APIs for (hwbinder) services of the

health HAL. If an implementation of the health HAL do not wish to provide any

storage info, include this library. Otherwise, it should implement the following

two functions:

void get_storage_info(std::vector<struct StorageInfo>& info) {

    // ...

}

void get_disk_stats(std::vector<struct DiskStats>& stats) {

    // ...

}