Android HIDL机制学习

前言：

在Android O之前，HAL是一个个的.so库，通过dlopen来进行打开，库和framework位于同一个进程。如下图所示：

在Android O之后，framework和hal运行于不同的进程，所有的HAL采用新的HIDL技术来完成。

一、HIDL简单介绍

HIDL是Android8.0新出的一个技能，以service和client的方式实现hal接口，目的是想使Android系统和BSP解绑，使系统升级更加方便。HIDL的使用方法一般是先提供.hal文件，然后使用hidl-gen工具生成 框架源文件和Android.bp编译工具文件，之后填充生成的源文件和定制Android.bp编译文件
官方介绍

二、以WiFi获取 MacAddress 为例看一下谷歌的实现

这是一个完整的 HIDL 接口的实现：
/frameworks/opt/net/wifi/service/java/com/android/server/wifi/WifiNative.java

/**
 * Makes a callback to HIDL to getMacAddress from supplicant
 *
 * @param ifaceName Name of the interface.
 * @return string containing the MAC address, or null on a failed call
 */
public String getMacAddress(@NonNull String ifaceName) {
    return mSupplicantStaIfaceHal.getMacAddress(ifaceName);
}

/frameworks/opt/net/wifi/service/java/com/android/server/wifi/SupplicantStaIfaceHal.java

/**
 * Makes a callback to HIDL to getMacAddress from supplicant
 *
 * @param ifaceName Name of the interface.
 * @return string containing the MAC address, or null on a failed call
 */
public String getMacAddress(@NonNull String ifaceName) {
    synchronized (mLock) {
        final String methodStr = "getMacAddress";
        ISupplicantStaIface iface = checkSupplicantStaIfaceAndLogFailure(ifaceName, methodStr);
        if (iface == null) return null;
        Mutable<String> gotMac = new Mutable<>();
        try {
            iface.getMacAddress((SupplicantStatus status,
                    byte[/* 6 */] macAddr) -> {
                if (checkStatusAndLogFailure(status, methodStr)) {
                    gotMac.value = NativeUtil.macAddressFromByteArray(macAddr);
                }
            });
        } catch (RemoteException e) {
            handleRemoteException(e, methodStr);
        }
        return gotMac.value;
    }
}

/hardware/interfaces/wifi/supplicant/1.0/ISupplicantStaIface.hal

/**
 * Send driver command to get MAC address of the device.
 *
 * @return status Status of the operation.
 *         Possible status codes:
 *         |SupplicantStatusCode.SUCCESS|,
 *         |SupplicantStatusCode.FAILURE_UNKNOWN|,
 *         |SupplicantStatusCode.FAILURE_IFACE_INVALID|
 * @return macAddr MAC address of the device.
 */
getMacAddress()
    generates (SupplicantStatus status, MacAddress macAddr);

/external/wpa_supplicant_8/wpa_supplicant/hidl/1.0/sta_iface.cpp

Return<void> StaIface::getMacAddress(getMacAddress_cb _hidl_cb)
{
	return validateAndCall(
	    this, SupplicantStatusCode::FAILURE_IFACE_INVALID,
	    &StaIface::getMacAddressInternal, _hidl_cb);
}

StaIface::getMacAddressInternal()
{
	struct wpa_supplicant *wpa_s = retrieveIfacePtr();
	std::vector<char> cmd(
	    kGetMacAddress, kGetMacAddress + sizeof(kGetMacAddress));
	char driver_cmd_reply_buf[4096] = {};
	int ret = wpa_drv_driver_cmd(
	    wpa_s, cmd.data(), driver_cmd_reply_buf,
	    sizeof(driver_cmd_reply_buf));
	// Reply is of the format: "Macaddr = XX:XX:XX:XX:XX:XX"
	std::string reply_str = driver_cmd_reply_buf;
	if (ret < 0 || reply_str.empty() ||
	    reply_str.find("=") == std::string::npos) {
		return {{SupplicantStatusCode::FAILURE_UNKNOWN, ""}, {}};
	}
	// Remove all whitespace first and then split using the delimiter "=".
	reply_str.erase(
	    remove_if(reply_str.begin(), reply_str.end(), isspace),
	    reply_str.end());
	std::string mac_addr_str =
	    reply_str.substr(reply_str.find("=") + 1, reply_str.size());
	std::array<uint8_t, 6> mac_addr;
	if (hwaddr_aton(mac_addr_str.c_str(), mac_addr.data())) {
		return {{SupplicantStatusCode::FAILURE_UNKNOWN, ""}, {}};
	}
	return {{SupplicantStatusCode::SUCCESS, ""}, mac_addr};
}