| <html devsite> |
| <head> |
| <title>Implementing USB HAL</title> |
| <meta name="project_path" value="/_project.yaml" /> |
| <meta name="book_path" value="/_book.yaml" /> |
| </head> |
| <body> |
| <!-- |
| Copyright 2017 The Android Open Source Project |
| |
| Licensed under the Apache License, Version 2.0 (the "License"); |
| you may not use this file except in compliance with the License. |
| You may obtain a copy of the License at |
| |
| http://www.apache.org/licenses/LICENSE-2.0 |
| |
| Unless required by applicable law or agreed to in writing, software |
| distributed under the License is distributed on an "AS IS" BASIS, |
| WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. |
| See the License for the specific language governing permissions and |
| limitations under the License. |
| --> |
| |
| <p> |
| The Android O release moves handling of USB commands out of <code>init</code> |
| scripts and into a native USB daemon for better configuration and code |
| reliability. For the Gadget function configuration, <code>init</code> scripts |
| (property triggers) are used to perform device-specific gadget operations. |
| </p> |
| |
| <p> |
| In previous releases, these device-specific configurations were achieved through |
| device-specific <code>init</code> scripts (using property triggers). Moving to a |
| Hardware Abstraction Layer (HAL) design results in a much cleaner implementation |
| that solves these problems: |
| </p> |
| |
| <ol> |
| <li>Operations such as writes to the kernel sysfs nodes could fail but not be |
| propagated back to the frameworks code that sets the property trigger. As a |
| result, frameworks incorrectly assumes the operations have succeeded even though |
| they have silently failed.</li> |
| <li><code>init</code> scripts have a limited number of operations that could be |
| executed.</li> |
| </ol> |
| |
| <h2 id="hal-and-treble">HAL and Treble</h2> |
| |
| <p> |
| The device-specific <code>init</code> scripts were used as a substitution for |
| HAL layers to perform device-specific USB operations. USB (through ADB) is a |
| primary interface for debugging system issues. Having a native daemon to perform |
| USB configuration eliminates the dependency on the framework code so even if the |
| framework crashes USB should be running. |
| </p> |
| |
| <p> |
| Under the <a |
| href="https://android-developers.googleblog.com/2017/05/here-comes-treble-modular-base-for.html">Treble</a> |
| model also introduced in Android O, all of the HALs are isolated from System |
| services and are required to run in their own native daemons. This eliminates |
| the requirement to have an exclusive USB daemon as the HAL layer nicely doubles |
| as a USB daemon. |
| </p> |
| |
| <p> |
| The default HAL implementation takes care of all pre-O devices. Therefore, there |
| wouldn't be any device-specific work for the pre-O devices. O uses the HAL |
| interface to query the status of USB ports and to perform data role and power |
| role swaps. |
| </p> |
| |
| <h2 id="implementation">Implementation</h2> |
| |
| <p> |
| New USB HAL interface needs to be implemented on every device launching on O. |
| The default implementation should take care of Pre-O devices. The default |
| implementation is sufficient if the device uses the <code>dual_role_usb</code> class to report |
| type-c port status. Trivial changes might be required in device-specific USB scripts |
| to transfer ownership of the typc-c nodes to system. |
| </p> |
| |
| <p> |
| No new platform APIs are introduced. |
| </p> |
| |
| <h2 id="validation">Validation</h2> |
| |
| <p> |
| VTS will test the new USB HAL. |
| </p> |
| |
| </body> |
| </html> |
