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127 lines
6.9 KiB
127 lines
6.9 KiB
4 months ago
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# NNAPI Conversions
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`convert` fails if either the source type or the destination type is invalid, and it yields a valid
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object if the conversion succeeds. For example, let's say that an enumeration in the current version
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has fewer possible values than the "same" canonical enumeration, such as `OperationType`. The new
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value of `HARD_SWISH` (introduced in Android R / NN HAL 1.3) does not map to any valid existing
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value in `OperationType`, but an older value of `ADD` (introduced in Android OC-MR1 / NN HAL 1.0) is
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valid. This can be seen in the following model conversions:
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```cpp
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// Unsuccessful conversion
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const nn::Model canonicalModel = createModelWhichHasV1_3Operations();
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const nn::Result<V1_0::Model> maybeVersionedModel = V1_0::utils::convert(canonicalModel);
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EXPECT_FALSE(maybeVersionedModel.has_value());
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```
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```cpp
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// Successful conversion
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const nn::Model canonicalModel = createModelWhichHasOnlyV1_0Operations();
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const nn::Result<V1_0::Model> maybeVersionedModel = V1_0::utils::convert(canonicalModel);
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ASSERT_TRUE(maybeVersionedModel.has_value());
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const V1_0::Model& versionedModel = maybeVersionedModel.value();
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EXPECT_TRUE(V1_0::utils::valid(versionedModel));
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```
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`V1_X::utils::convert` does not guarantee that all information is preserved. For example, In the
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case of `nn::ErrorStatus`, the new value of `MISSED_DEADLINE_TRANSIENT` can be represented by the
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existing value of `V1_0::GENERAL_FAILURE`:
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```cpp
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// Lossy Canonical -> HAL -> Canonical conversion
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const nn::ErrorStatus canonicalBefore = nn::ErrorStatus::MISSED_DEADLINE_TRANSIENT;
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const V1_0::ErrorStatus versioned = V1_0::utils::convert(canonicalBefore).value();
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const nn::ErrorStatus canonicalAfter = nn::convert(versioned).value();
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EXPECT_NE(canonicalBefore, canonicalAfter);
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```
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However, `nn::convert` is guaranteed to preserve all information:
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```cpp
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// Lossless HAL -> Canonical -> HAL conversion
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const V1_0::ErrorStatus versionedBefore = V1_0::ErrorStatus::GENERAL_FAILURE;
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const nn::ErrorStatus canonical = nn::convert(versionedBefore).value();
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const V1_0::ErrorStatus versionedAfter = V1_0::utils::convert(canonical).value();
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EXPECT_EQ(versionedBefore, versionedAfter);
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```
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The `convert` functions operate only on types that used in a HIDL method call directly. The
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`unvalidatedConvert` functions operate on types that are either used in a HIDL method call directly
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(i.e., not as a nested class) or used in a subsequent version of the NN HAL. Prefer using `convert`
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over `unvalidatedConvert`.
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# Interface Lifetimes across Processes
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## HIDL
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Some notes about HIDL interface objects and lifetimes across processes:
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All HIDL interface objects inherit from `IBase`, which itself inherits from `::android::RefBase`. As
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such, all HIDL interface objects are reference counted and must be owned through `::android::sp` (or
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referenced through `::android::wp`). Allocating `RefBase` objects on the stack will log errors and
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may result in crashes, and deleting a `RefBase` object through another means (e.g., "delete",
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"free", or RAII-cleanup through `std::unique_ptr` or some equivalent) will result in double-free
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and/or use-after-free undefined behavior.
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HIDL/Binder manages the reference count of HIDL interface objects automatically across processes. If
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a process that references (but did not create) the HIDL interface object dies, HIDL/Binder ensures
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any reference count it held is properly released. (Caveat: it might be possible that HIDL/Binder
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behave strangely with `::android::wp` references.)
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If the process which created the HIDL interface object dies, any call on this object from another
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process will result in a HIDL transport error with the code `DEAD_OBJECT`.
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## AIDL
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We use NDK backend for AIDL interfaces. Handling of lifetimes is generally the same with the
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following differences:
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* Interfaces inherit from `ndk::ICInterface`, which inherits from `ndk::SharedRefBase`. The latter
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is an analog of `::android::RefBase` using `std::shared_ptr` for reference counting.
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* AIDL calls return `ndk::ScopedAStatus` which wraps fields of types `binder_status_t` and
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`binder_exception_t`. In case the call is made on a dead object, the call will return
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`ndk::ScopedAStatus` with exception `EX_TRANSACTION_FAILED` and binder status
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`STATUS_DEAD_OBJECT`.
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# Protecting Asynchronous Calls
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## Across HIDL
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Some notes about asynchronous calls across HIDL:
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For synchronous calls across HIDL, if an error occurs after the function was called but before it
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returns, HIDL will return a transport error. For example, if the message cannot be delivered to the
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server process or if the server process dies before returning a result, HIDL will return from the
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function with the appropriate transport error in the `Return<>` object, which can be queried with
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`Return<>::isOk()`, `Return<>::isDeadObject()`, `Return<>::description()`, etc.
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However, HIDL offers no such error management in the case of asynchronous calls. By default, if the
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client launches an asynchronous task and the server fails to return a result through the callback,
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the client will be left waiting indefinitely for a result it will never receive.
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In the NNAPI, `IDevice::prepareModel*` and `IPreparedModel::execute*` (but not
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`IPreparedModel::executeSynchronously*`) are asynchronous calls across HIDL. Specifically, these
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asynchronous functions are called with a HIDL interface callback object (`IPrepareModelCallback` for
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`IDevice::prepareModel*` and `IExecutionCallback` for `IPreparedModel::execute*`) and are expected
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to quickly return, and the results are returned at a later time through these callback objects.
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To protect against the case when the server dies after the asynchronous task was called successfully
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but before the results could be returned, HIDL provides an object called a "`hidl_death_recipient`,"
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which can be used to detect when an interface object (and more generally, the server process) has
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died. nnapi/hal/ProtectCallback.h's `DeathHandler` uses `hidl_death_recipient`s to detect when the
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driver process has died, and `DeathHandler` will unblock any thread waiting on the results of an
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`IProtectedCallback` callback object that may otherwise not be signaled. In order for this to work,
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the `IProtectedCallback` object must have been registered via `DeathHandler::protectCallback()`.
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## Across AIDL
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We use NDK backend for AIDL interfaces. Handling of asynchronous calls is generally the same with
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the following differences:
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* AIDL calls return `ndk::ScopedAStatus` which wraps fields of types `binder_status_t` and
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`binder_exception_t`. In case the call is made on a dead object, the call will return
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`ndk::ScopedAStatus` with exception `EX_TRANSACTION_FAILED` and binder status
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`STATUS_DEAD_OBJECT`.
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* AIDL interface doesn't contain asynchronous `IPreparedModel::execute`.
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* Service death is handled using `AIBinder_DeathRecipient` object which is linked to an interface
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object using `AIBinder_linkToDeath`. nnapi/hal/aidl/ProtectCallback.h provides `DeathHandler`
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object that is a direct analog of HIDL `DeathHandler`, only using libbinder_ndk objects for
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implementation.
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