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diff --git a/en/devices/architecture/configstore/index.html b/en/devices/architecture/configstore/index.html new file mode 100644 index 00000000..45dcb9e7 --- /dev/null +++ b/en/devices/architecture/configstore/index.html @@ -0,0 +1,108 @@ +<html devsite> + <head> + <title>Configstore 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>Android O splits the monolithic Android OS into generic (system.img) +and hardware-specific (vendor.img and odm.img) partitions. As a result of this +change, conditional compilation must be removed from modules installed to the +system partition and such modules must now determine the configuration of the +system at runtime (and behave differently depending on that configuration).</p> + +<p>The ConfigStore HAL provides a set of APIs for accessing read-only +configuration items used to configure the Android framework. This page describes +the design of ConfigStore HAL (and why system properties were not used for this +purpose); other pages in this section detail the +<a href="/devices/architecture/configstore/interface.html">HAL interface</a>, +<a href="/devices/architecture/configstore/service.html">service +implementation</a>, and +<a href="/devices/architecture/configstore/client.html">client-side usage</a>, +all using <code>surfaceflinger</code> as an example. For help with ConfigStore +interface classes, see +<a href="/devices/architecture/configstore/add-class-item.html">Adding Interface +Classes & Items</a>.</p> + +<h2 id=system-properties>Why not use system properties?</h2> +<p>We considered using system properties but found several fundamental issues, +including: </p> +<ul> +<li><strong>Length limits on values</strong>. System properties have +tight limits on the length of their values (92 bytes). In addition, as these +limits have been directly exposed to Android apps as C macros, increasing the +length can cause backwards-compatibility issues.</li> +<li><strong>No type support</strong>. All values are essentially strings, and +APIs simply parse the string into an <code>int</code> or <code>bool</code>. +Other compound data types (array, struct, etc.) should be encoded/decoded by +the clients (e.g. "aaa,bbb,ccc" can be decoded as an array of three strings). +</li> +<li><strong>Overwrites</strong>. Because read-only system properties are +implemented as write-once properties, vendors/ODMs that want to override +AOSP-defined read-only values must import their own read-only values prior to +AOSP-defined read-only values, which in turn results in vendor-defined +re-writable values being overridden by AOSP-defined values.</li> +<li><strong>Address space requirements</strong>. System properties take a +relatively large amount of address space in each process. System properties are +grouped in <code>prop_area</code> units with a fixed size of 128KB, all of which +is allocated to a process address space even if only a single system property in +it is being accessed. This can cause problems on 32-bit devices where address +space is precious.</li> +</ul> +<p>We attempted to overcome these limitations without sacrificing compatibility +but continued to be concerned that system properties were not designed to +support accessing read-only configuration items. Eventually we decided that +system properties are better suited for sharing a few dynamically-updated items +across all of Android in real time, and that a need existed for a new system +dedicated to accessing read-only configuration items.</p> + +<h2>ConfigStore HAL design</h2> +<p>The basic design is simple:</p> +<p><img src="../images/treble_configstore_design.png"></p> +<p><strong>Figure 1.</strong> ConfigStore HAL design</p> + +<ul> +<li>Describe build flags (currently used for conditionally compiling the +framework) in HIDL.</li> +<li>Vendors and OEMs provide SoC and device-specific values for build flags by +implementing the HAL service.</li> +<li>Modify the framework to use the HAL service to find the value of a +configuration item at runtime.</li> +</ul> + +<p>Configuration items currently referenced by the framework are included in a +versioned HIDL package (<code>android.hardware.configstore@1.0</code>). Vendors +and/or OEMs provide values to the configuration items by implementing interfaces +in this package, and the framework uses the interfaces when it needs to get a +value for a configuration item.</p> + +<h2 id=security>Security considerations</h2> +<p>Build flags defined in the same interface are affected by same SELinux +policy. If one or more build flags should have different SELinux policies, +<strong>they must be separated to another interface</strong>. This can require +major uprev of <code>android.hardware.configstore package</code> as the +separated interfaces are no longer backwards-compatible.</p> + +<aside class="note"><strong>Note:</strong> For details on Android 8.0 SELinux, +see <a href="/security/selinux/images/SELinux_Treble.pdf">SELinux for Android +8.0</a>.</aside> + + </body> +</html> |