This document describes HIDL interface hashing, a mechanism to prevent accidental interface changes and ensure interface changes are thoroughly vetted. This mechanism is required because HIDL interfaces are versioned, which means that after an interface is released it must not be changed except in an Application Binary Interface (ABI) preserving manner (such as a comment correction).
Every package root directory (i.e.
android.hardware mapping to
vendor.foo mapping to
vendor/foo/hardware/interfaces) must contain a
current.txt file that lists all released HIDL interface files.
# current.txt files support comments starting with a ‘#' character # this file, for instance, would be vendor/foo/hardware/interfaces/current.txt # Each line has a SHA-256 hash followed by the name of an interface. # They have been shortened in this doc for brevity but they are # 64 characters in length in an actual current.txt file. d4ed2f0e...995f9ec4 firstname.lastname@example.org::IFoo # comments can also go here # types.hal files are also noted in current.txt files c84da9f5...f8ea2648 email@example.com::types # Multiple hashes can be in the file for the same interface. This can be used # to note how ABI sustaining changes were made to the interface. # For instance, here is another hash for IFoo: # Fixes type where "FooCallback" was misspelled in comment on "FooStruct" 822998d7...74d63b8c firstname.lastname@example.org::IFoo
Note: To help keep track of which hashes come
from where, Google separates HIDL
current.txt files into different
sections: The first section is Released in Android O; the next section
will be Released in Android O MR1. We strongly recommend using a
similar layout in your
Hashing with hidl-gen
You can add a hash to a
current.txt file manually or by
hidl-gen. The following code snippet provides examples of
commands you can use with
hidl-gen to manage a
current.txt file (hashes have been shortened):
hidl-gen -L hash -r vendor.awesome:vendor/awesome/hardware/interfaces -r android.hardware:hardware/interfaces -r android.hidl:system/libhidl/transport email@example.com::types9626fd18...f9d298a6 firstname.lastname@example.org::types
hidl-gen -L hash -r vendor.awesome:vendor/awesome/hardware/interfaces -r android.hardware:hardware/interfaces -r android.hidl:system/libhidl/transport email@example.com::INfc07ac2dc9...11e3cf57 firstname.lastname@example.org::INfc
hidl-gen -L hash -r vendor.awesome:vendor/awesome/hardware/interfaces -r android.hardware:hardware/interfaces -r android.hidl:system/libhidl/transport email@example.com...f9d298a6 firstname.lastname@example.org::types 07ac2dc9...11e3cf57 email@example.com::INfc f2fe5442...72655de6 firstname.lastname@example.org::INfcClientCallback
hidl-gen -L hash -r vendor.awesome:vendor/awesome/hardware/interfaces -r android.hardware:hardware/interfaces -r android.hidl:system/libhidl/transport email@example.com >> vendor/awesome/hardware/interfaces/current.txt
Warning: Do not replace a hash for a
previously-released interface. When changing such an interface, add a new hash
to the end of the
current.txt file. For details, refer to
Every interface definition library generated by
includes hashes, which can be retrieved by calling
hidl-gen is compiling an
interface, it checks the
current.txt file in the root directory of
the HAL package to see if the HAL has been changed:
- If no hash for the HAL is found, the interface is considered unreleased (in development) and compilation proceeds.
- If hashes are found, they are checked against the current interface:
- If the interface does match the hash, compilation proceeds.
- If the interface does not match a hash, compilation is halted as this means a previously-released interface is being changed.
- For an ABI-preserving change (see
ABI stability), the
current.txtfile must be modified before compilation can proceed.
- All other changes should be made in a minor or major version upgrade of the interface.
- For an ABI-preserving change (see ABI stability), the
An Application Binary Interface (ABI) includes the binary
linkages/calling conventions/etc. If the ABI/API changes, the interface no
longer works with a generic
system.img that was compiled with
Making sure that interfaces are versioned and ABI stable is crucial for several reasons:
- It ensures your implementation can pass the Vendor Test Suite (VTS), which puts you on track to being able to do framework-only OTAs.
- As an OEM, it enables you to provide a Board Support Package (BSP) that is straightforward to use and compliant.
- It helps you keep track of what interfaces can be released. Consider
current.txta map of an interfaces directory that allows you to see the history and state of all interfaces being provided in a package root.
When adding a new hash for an interface that already has an entry in
current.txt, make sure to add only the hashes that represent
interfaces which maintain ABI stability. Review the following types of changes:
|Changes not allowed||