Aidl for Hal

Android 11 Introduced in Android Use in AIDL Realization HAL The function of . So you can use it without HIDL In the case of Android Part of the code . We strongly recommend that you try to HAL Convert to use only AIDL（ If upstream HAL Use HIDL, Must be used HIDL）.

If HAL Use AIDL In frame assembly （ for example  system.img  Components in ） And hardware components （ for example  vendor.img  Components in ） Communicate with each other , You must use stable AIDL. however , If you need to communicate in the partition （ For example, from a HAL To another ）, For the used IPC There are no restrictions on the mechanism .

motivation

AIDL Appear in the HIDL Before , And in Android It is used in many other places, such as between framework components or in applications . Now? , because AIDL With stability support , You can use only one IPC The runtime environment implements the entire stack . Besides ,AIDL The version control system of is also better than HIDL.

• Use only one IPC Language means only knowing 、 debugging 、 Optimize and protect a runtime environment .
• AIDL It can provide the interface owner with a built-in version control mechanism .
  • Owners can add methods to Parcelable End of interface or field of . This means that version control of code can be simplified in the development process that lasts for many years , And reduce the cost year by year （ You can modify the type in place , And there is no need to add additional libraries to update the interface version ）.
  • Extension interfaces can be attached at run time , Instead of appending , Therefore, there is no need to expand downstream rebase To the new interface .
• If existing AIDL The owner of the interface chooses to stabilize it , This kind of interface can be used directly . If it was before , Must use HIDL Create a complete copy of the interface .

To write AIDL HAL Interface

In order for the system and suppliers to use a AIDL Interface , Two changes need to be made to this interface ：

• Every type definition needs to have  @VintfStability  notes
• aidl_interface  The declaration needs to contain  stability: "vintf",

Only the interface owner can make these changes .

Besides , To maximize code portability and avoid potential problems （ For example, unnecessary additional libraries ）, It is recommended that you disable CPP Back end .

    aidl_interface: {
        ...
        backends: {
            cpp: {
                enabled: false,
            },
        },
    }

lookup AIDL HAL Interface

AOSP in HAL The stability of the AIDL The basic directory where the interface is located is the same as HIDL The basic directory of the interface is the same , be located  aidl  In the folder .

• hardware/interfaces
• frameworks/hardware/interfaces
• system/hardware/interfaces

You should put the extension interface into  vendor  or  hardware  In the other  hardware/interfaces  subdirectories . It is strongly recommended that you maintain a consistent interface between all devices . Extensions can be registered in two different ways ：

• Register at runtime
• Independent registration （ Register globally and at VINTF Internal registration ）

in the light of AIDL Build the runtime environment

AIDL There are three different back ends ：Java、NDK、CPP. If you need to use stable AIDL, You must always use at  system/lib*/libbinder.so  Of libbinder System copy of , And be able to access  /dev/binder. For the code in the vendor image , It means  libbinder（ come from VNDK） Can't use ： This library contains unstable C++ API And unstable internals . The native vendor code must use AIDL Of NDK Back end , link to  libbinder_ndk（ By system  libbinder.so  Provide support ）, And link to by  aidl_interface  Items created  -ndk_platform  library .

AIDL HAL Server instance name

By convention ,AIDL HAL The instance name of the service should be in the following format ：$package.$type/$instance. for example , Vibrator HAL The registered name of the instance is  android.hardware.vibrator.IVibrator/default.

To write AIDL HAL The server

Must be in VINTF The manifest States AIDL The server , Examples are as follows ：

    <hal format="aidl">
        <name>android.hardware.vibrator</name>
        <fqname>IVibrator/default</fqname>
    </hal>

otherwise , They should be registered normally AIDL service . function VTS When testing , All declared AIDL HAL Should be available

To write AIDL client

AIDL The client must declare itself in the compatibility matrix , Examples are as follows ：

    <name>android.hardware.vibrator</name>
    <interface>
        <name>IVibrator</name>
        <instance>default</instance>
    </interface>

Put the existing HAL from HIDL Convert to AIDL

Use  hidl2aidl  Tools will HIDL Interface to AIDL. Follow the steps below , take .hal The package is converted to .aidl file ：

1. Build the tool （ be located  system/tools/hidl/hidl2aidl  in

   m hidl2aidl

2. Use the following command to execute the tool ： The output directory , Followed by the package to be converted  

   hidl2aidl -o <output directory> <package>

3. Read the generated file carefully , And solve any conversion problems .
   conversion.log  Include all unresolved issues that should be resolved first .
   Generated  .aidl  The file may contain some warnings and suggestions that you need to deal with accordingly . These notes are in  //  start .
   Please seize this opportunity , Clean up and improve the package .

Apply to AIDL HAL Of sepolicy

Visible to supplier code AIDL The service type must have  vendor_service  attribute . otherwise ,sepolicy Configuration will work with any other AIDL The service is the same

    type hal_power_service, service_manager_type, vendor_service;

For most services defined by the platform , A service context with the correct type has been added （ for example ,android.hardware.power.IPower/default  Has been marked as  hal_power_service）. however , If the framework client supports multiple instance names , Must be dedicated to the equipment  service_contexts  Add other instance names to the file

    android.hardware.power.IPower/custom_instance u:object_r:hal_power_service:s0

AIDL And HIDL The main difference between

Use AIDL HAL Or use AIDL HAL Interface , Please note and write HIDL HAL The difference of .

• AIDL The grammar of language is closer Java.HIDL The grammar of a language is similar to C++.
• all AIDL Interfaces have built-in error states . Do not create custom state types , Instead, create a constant state in the interface file int, And in CPP/NDK The back-end using  EX_SERVICE_SPECIFIC, stay Java The back-end using  ServiceSpecificException.
• Unchecked transmission errors do not cause AIDL Terminate run （ Unchecked errors can lead to HIDL Return  Terminate run ）.
• AIDL Only one type can be declared for each file .
• except output Out of parameters ,AIDL Parameters can also be specified as in/out/inout（ No, “ Synchronous callback ”）.
• AIDL take fd Used as primitive type , Not the handle .
• HIDL Use for incompatible changes Major edition , Use for compatible changes Minor edition . stay AIDL in , Backward compatible changes are in place . AIDL No, right Major The version is clearly defined , Instead, it is incorporated into the package name . for example ,AIDL The package name may be used  bluetooth2.