Camera HAL3 buffer management APIs

Android 10 introduces optional camera HAL3 buffer management APIs that allow you to implement buffer management logic to achieve different memory and capture latency tradeoffs in camera HAL implementations.

The camera HAL requires N requests (where N is equal to the pipeline depth) queued in its pipeline, but it often doesn't require all N sets of output buffers at the same time.

For example, the HAL might have eight requests queued in the pipeline, but it only requires output buffers for the two requests in the last stages of the pipeline. On devices running Android 9 and lower, the camera framework allocates buffers when the request is queued in the HAL so there could be six sets of buffers in the HAL that aren't in use. In Android 10, the camera HAL3 buffer management APIs allow for the decoupling of the output buffers to free up the six sets of buffers. This can lead to hundreds of megabytes of memory savings on high-end devices and can also be beneficial for low-memory devices.

Figure 1 shows a diagram of the camera HAL interface for devices running Android 9 and lower. Figure 2 shows the camera HAL interface in Android 10 with the camera HAL3 buffer management APIs implemented.

Figure 1. Camera HAL interface in Android 9 and lower

Figure 2. Camera HAL interface in Android 10 using the buffer management APIs

Implement the buffer management APIs

To implement the buffer management APIs, the camera HAL must:

• Implement HIDL ICameraDevice@3.5.
• Set the camera characteristics key android.info.supportedBufferManagementVersion to HIDL_DEVICE_3_5.

The camera HAL uses the requestStreamBuffers and returnStreamBuffers methods in ICameraDeviceCallback.hal to request and return buffers. The HAL must also implement the signalStreamFlush method in ICameraDeviceSession.hal to signal the camera HAL to return buffers.

Note: If the camera HAL implements these buffer management APIs on a device running Android 9 and lower, the HAL is required to support the API contract illustrated in Figure 1.

requestStreamBuffers

Use the requestStreamBuffers method to request buffers from the camera framework. When using the camera HAL3 buffer management APIs, capture requests from the camera framework don't contain output buffers, that is, the bufferId field in StreamBuffer is 0. Therefore, the camera HAL must use requestStreamBuffers to request buffers from the camera framework.

The requestStreamBuffers method allows the caller to request multiple buffers from multiple output streams in a single call, allowing for fewer HIDL IPC calls. However, calls take more time when more buffers are requested at the same time and this might negatively affect the total request-to-result latency. Also, because calls into requestStreamBuffers are serialized in the camera service, it's recommended that the camera HAL use a dedicated high-priority thread to request buffers.

If a buffer request fails, the camera HAL must be able to properly handle nonfatal errors. The following list describes common reasons that buffer requests fail and how they should be handled by the camera HAL.

• App disconnects from the output stream: This is a nonfatal error. The camera HAL should send ERROR_REQUEST for any capture request targeting a disconnected stream and be ready to process subsequent requests normally.
• Timeout: This can occur when an app is busy doing intensive processing while holding onto some buffers. The camera HAL should send ERROR_REQUEST for capture requests that can't be fulfilled due to a timeout error and be ready to process subsequent requests normally.
• Camera framework is preparing a new stream configuration: The camera HAL should wait until the next configureStreams call is complete before calling requestStreamBuffers again.
• The camera HAL has reached its buffer limit (the maxBuffers field): The camera HAL should wait until it returns at least one buffer of the stream before calling requestStreamBuffers again.

returnStreamBuffers

Use the returnStreamBuffers method to return extra buffers to the camera framework. The camera HAL normally returns buffers to the camera framework through the processCaptureResult method, but it can only account for capture requests that have been sent to the camera HAL. With the requestStreamBuffers method, it's possible for the camera HAL implementation to retain more buffers than what has been requested by the camera framework. This is when the returnStreamBuffers method should be used. If the HAL implementation never holds more buffers than requested, the camera HAL implementation doesn't need to call the returnStreamBuffers method.

signalStreamFlush

The signalStreamFlush method is called by the camera framework to notify the camera HAL to return all buffers at hand. This is normally called when the camera framework is about to call configureStreams and must drain the camera capture pipeline. Similar to the returnStreamBuffers method, if a camera HAL implementation doesn't hold more buffers than requested, it's possible to have an empty implementation of this method.

After the camera framework calls signalStreamFlush, the framework stops sending new capture requests to the camera HAL until all buffers have been returned to the camera framework. When all buffers are returned, the requestStreamBuffers method calls fail, and the camera framework can continue its work in a clean state. The camera framework then calls either the configureStreams or processCaptureRequest method. If the camera framework calls the configureStreams method, the camera HAL can start requesting buffers again after the configureStreams call returns successfully. If the camera framework calls the processCaptureRequest method, the camera HAL can start requesting buffers during the processCaptureRequest call.

The semantics are different for the signalStreamFlush method and the flush method. When the flush method is called, the HAL can abort pending capture requests with ERROR_REQUEST to drain the pipeline as soon as possible. When the signalStreamFlush method is called, the HAL must finish all pending capture requests normally and return all buffers to the camera framework.

Another difference between the signalStreamFlush method and other methods is that signalStreamFlush is a one-way HIDL method, which means that the camera framework might call into other blocking APIs before the HAL receives the signalStreamFlush call. This means that the signalStreamFlush method and other methods (specifically the configureStreams method) might arrive at the camera HAL in a different order than the order they were called in the camera framework. To address this asynchrony issue, the streamConfigCounter field was added to StreamConfiguration and added as an argument to the signalStreamFlush method. The camera HAL implementation should use the streamConfigCounter argument to determine whether a signalStreamFlush call arrives later than its corresponding configureStreams call. See Figure 3 for an example.

Figure 3. How the camera HAL should detect and handle signalStreamFlush calls that arrive late

Behavior changes when implementing the buffer management APIs

When using the buffer management APIs to implement the buffer management logic, consider the following possible behavior changes to the camera and camera HAL implementation:

• Capture requests arrive at the camera HAL faster and more frequently: Without buffer management APIs, the camera framework requests output buffers for each capture request before sending a capture request to the camera HAL. When using the buffer management APIs, the camera framework no longer needs to wait for buffers and can therefore send capture requests to the camera HAL earlier.

  Also, without buffer management APIs, the camera framework stops sending capture requests if one of the output streams of the capture request has reached the maximum number of buffers that the HAL can hold at one time (this value is designated by the camera HAL in the HalStream::maxBuffers field in the return value of a configureStreams call). With the buffer management APIs, this throttling behavior no longer exists and the camera HAL implementation must not accept processCaptureRequest calls when the HAL has too many capture requests queued.

• requestStreamBuffers call latency varies significantly: There are many reasons a requestStreamBuffers call might take a longer time than average. For example:

  • For the first few buffers of a newly created stream, calls can take longer because the device needs to allocate memory.
  • The expected latency increases in proportion to the number of buffers requested in each call.
  • The app is holding buffers and is busy processing. This can cause buffer requests to slow down or hit a timeout because of a lack of buffers or a busy CPU.

Buffer management strategies

The buffer management APIs allow for different kinds of buffer management strategies to be implemented. Some examples are:

• Backward compatible: The HAL requests buffers for a capture request during the processCaptureRequest call. This strategy doesn't provide any memory savings, but can serve as the first implementation of the buffer management APIs, requiring very few code changes to the existing camera HAL.
• Maximized memory savings: The camera HAL only requests output buffers immediately before one is needed to be filled. This strategy allows for maximized memory savings. The potential downside is more camera pipeline jank when buffer requests take an unusually long time to finish.
• Cached: The camera HAL caches a few buffers so that it's less likely to be affected by an occasional slow buffer request.

The camera HAL can adopt different strategies for particular use cases, for example, using the maximized memory saving strategy for use cases that use a lot of memory and using the backward-compatible strategy for other use cases.

Sample implementation in the external camera HAL

The external camera HAL was introduced in Android 9 and can be found in the source tree at hardware/interfaces/camera/device/3.5/. In Android 10, it has been updated to include ExternalCameraDeviceSession.cpp, an implementation of the buffer management API. This external camera HAL implements the maximized memory savings strategy mentioned in Buffer management strategies in a few hundred lines of C++ code.