背景
无论是SmartTV还是SmartPhone，IC厂商一般会有MultiMedia的HardWare encode\decode实现，当这些IC要跑Android时，就要把各自的实现对接到Android MediaPlayer Framework中，而Android MediaPlayer Codec部分是通过OMX框架实现的。
OMXMaster
OMXMaster的角色，OMXPlugin的管理者，负责软解码和厂商硬件解码的维护。先看看OMXMaster的构造函数：
OMXMaster::OMXMaster()
: mVendorLibHandle(NULL) {
addVendorPlugin();
addPlugin(new SoftOMXPlugin);
}
addVenderPlugin用于添加厂商自己的Codec，SoftOMXPlugin通过addPlugin添加。我们这里关注厂商要添加自己OMXPlugin decode的流程：
void OMXMaster::addVendorPlugin() {
addPlugin("libstagefrighthw.so");
}
函数的实现很简单，调用addPlugin，把一个共享库的名字作为参数传入。从这里大概可以猜到，厂商私有的codec实现会封装在动态共享库中。接下来继续看下addPlugin的实现：
void OMXMaster::addPlugin(const char *libname) {
mVendorLibHandle = dlopen(libname, RTLD_NOW);
if (mVendorLibHandle == NULL) {
return;
}
typedef OMXPluginBase *(*CreateOMXPluginFunc)();
CreateOMXPluginFunc createOMXPlugin =
(CreateOMXPluginFunc)dlsym(
mVendorLibHandle, "createOMXPlugin");
if (!createOMXPlugin)
createOMXPlugin = (CreateOMXPluginFunc)dlsym(
mVendorLibHandle, "_ZN7android15createOMXPluginEv");
if (createOMXPlugin) {
addPlugin((*createOMXPlugin)());
}
}
函数的实现中首先看到通过dlopen打开共享库，并通过dlsym找到createOMXPlugin函数symbol，然后调用createOMXPlugin，返回OMXPluginBase类型的子类。addPlugin对应的是下面这个函数:
void OMXMaster::addPlugin(OMXPluginBase *plugin){
Mutex::Autolock autoLock(mLock);
mPlugins.push_back(plugin);
OMX_U32 index = 0;
char name[128];
OMX_ERRORTYPE err;
while ((err = plugin->enumerateComponents(
name, sizeof(name), index++)) == OMX_ErrorNone) {
String8 name8(name);
if (mPluginByComponentName.indexOfKey(name8) >= 0) {
ALOGE("A component of name '%s' already exists, ignoring this one.",
name8.string());
continue;
}
mPluginByComponentName.add(name8, plugin);
}
if (err != OMX_ErrorNoMore) {
ALOGE("OMX plugin failed w/ error 0x%08x after registering %d "
"components", err, mPluginByComponentName.size());
}
}
函数的实现通过while循环枚举该plugin中的所有Components,可以简单理解为一个Component对应一种Codec。
至此，厂商的Codec Component就添加进来，便可在OMX中供mediaplayer使用。
OMXPluginBase
如果要将自己的codec接入OMX框架要如何做呢？
需要继承OMXPluginBase类，并实现抽象类中的方法。
struct OMXPluginBase {
OMXPluginBase() {}
virtual ~OMXPluginBase() {}
virtual OMX_ERRORTYPE makeComponentInstance(
const char *name,
const OMX_CALLBACKTYPE *callbacks,
OMX_PTR appData,
OMX_COMPONENTTYPE **component) = 0;
virtual OMX_ERRORTYPE destroyComponentInstance(
OMX_COMPONENTTYPE *component) = 0;
virtual OMX_ERRORTYPE enumerateComponents(
OMX_STRING name,
size_t size,
OMX_U32 index) = 0;
virtual OMX_ERRORTYPE getRolesOfComponent(
const char *name,
Vector<String8> *roles) = 0;
private:
OMXPluginBase(const OMXPluginBase &);
OMXPluginBase &operator=(const OMXPluginBase &);
}
在C++中，是通过抽象类来模拟接口，所以OMXPluginBase是作为OMXPlugin的一种抽象，任何一种OMXPlugin要实现这个接口。这个抽象类中，存在4个需要实现的函数，makeComponentInstance，destroyComponentInstance，enumerateComponents，getRolesOfComponent。
1)makeComponentInstance
OMX_ERRORTYPE RTKOMXPlugin::makeComponentInstance(
const char *name,
const OMX_CALLBACKTYPE *callbacks,
OMX_PTR appData,
OMX_COMPONENTTYPE **component)
通过name，创建一个Codec,OMX_COMPONENTTYPE **component是输出的component，OMX_COMPONENTTYPE可以当做Codec的抽象，
typedef struct OMX_COMPONENTTYPE
{
/** The size of this structure, in bytes. It is the responsibility
of the allocator of this structure to fill in this value. Since
this structure is allocated by the GetHandle function, this
function will fill in this value. */
OMX_U32 nSize;
/** nVersion is the version of the OMX specification that the structure
is built against. It is the responsibility of the creator of this
structure to initialize this value and every user of this structure
should verify that it knows how to use the exact version of
this structure found herein. */
OMX_VERSIONTYPE nVersion;
/** pComponentPrivate is a pointer to the component private data area.
This member is allocated and initialized by the component when the
component is first loaded. The application should not access this
data area. */
OMX_PTR pComponentPrivate;
/** pApplicationPrivate is a pointer that is a parameter to the
OMX_GetHandle method, and contains an application private value
provided by the IL client. This application private data is
returned to the IL Client by OMX in all callbacks */
OMX_PTR pApplicationPrivate;
/** refer to OMX_GetComponentVersion in OMX_core.h or the OMX IL
specification for details on the GetComponentVersion method.
*/
OMX_ERRORTYPE (*GetComponentVersion)(
OMX_IN OMX_HANDLETYPE hComponent,
OMX_OUT OMX_STRING pComponentName,
OMX_OUT OMX_VERSIONTYPE* pComponentVersion,
OMX_OUT OMX_VERSIONTYPE* pSpecVersion,
OMX_OUT OMX_UUIDTYPE* pComponentUUID);
/** refer to OMX_SendCommand in OMX_core.h or the OMX IL
specification for details on the SendCommand method.
*/
OMX_ERRORTYPE (*SendCommand)(
OMX_IN OMX_HANDLETYPE hComponent,
OMX_IN OMX_COMMANDTYPE Cmd,
OMX_IN OMX_U32 nParam1,
OMX_IN OMX_PTR pCmdData);
/** refer to OMX_GetParameter in OMX_core.h or the OMX IL
specification for details on the GetParameter method.
*/
OMX_ERRORTYPE (*GetParameter)(
OMX_IN OMX_HANDLETYPE hComponent,
OMX_IN OMX_INDEXTYPE nParamIndex,
OMX_INOUT OMX_PTR pComponentParameterStructure);
/** refer to OMX_SetParameter in OMX_core.h or the OMX IL
specification for details on the SetParameter method.
*/
OMX_ERRORTYPE (*SetParameter)(
OMX_IN OMX_HANDLETYPE hComponent,
OMX_IN OMX_INDEXTYPE nIndex,
OMX_IN OMX_PTR pComponentParameterStructure);
/** refer to OMX_GetConfig in OMX_core.h or the OMX IL
specification for details on the GetConfig method.
*/
OMX_ERRORTYPE (*GetConfig)(
OMX_IN OMX_HANDLETYPE hComponent,
OMX_IN OMX_INDEXTYPE nIndex,
OMX_INOUT OMX_PTR pComponentConfigStructure);
/** refer to OMX_SetConfig in OMX_core.h or the OMX IL
specification for details on the SetConfig method.
*/
OMX_ERRORTYPE (*SetConfig)(
OMX_IN OMX_HANDLETYPE hComponent,
OMX_IN OMX_INDEXTYPE nIndex,
OMX_IN OMX_PTR pComponentConfigStructure);
/** refer to OMX_GetExtensionIndex in OMX_core.h or the OMX IL
specification for details on the GetExtensionIndex method.
*/
OMX_ERRORTYPE (*GetExtensionIndex)(
OMX_IN OMX_HANDLETYPE hComponent,
OMX_IN OMX_STRING cParameterName,
OMX_OUT OMX_INDEXTYPE* pIndexType);
/** refer to OMX_GetState in OMX_core.h or the OMX IL
specification for details on the GetState method.
*/
OMX_ERRORTYPE (*GetState)(
OMX_IN OMX_HANDLETYPE hComponent,
OMX_OUT OMX_STATETYPE* pState);
/** The ComponentTunnelRequest method will interact with another OMX
component to determine if tunneling is possible and to setup the
tunneling. The return codes for this method can be used to
determine if tunneling is not possible, or if tunneling is not
supported.
Base profile components (i.e. non-interop) do not support this
method and should return OMX_ErrorNotImplemented
The interop profile component MUST support tunneling to another
interop profile component with a compatible port parameters.
A component may also support proprietary communication.
If proprietary communication is supported the negotiation of
proprietary communication is done outside of OMX in a vendor
specific way. It is only required that the proper result be
returned and the details of how the setup is done is left
to the component implementation.
When this method is invoked when nPort in an output port, the
component will:
1. Populate the pTunnelSetup structure with the output port's
requirements and constraints for the tunnel.
When this method is invoked when nPort in an input port, the
component will:
1. Query the necessary parameters from the output port to
determine if the ports are compatible for tunneling
2. If the ports are compatible, the component should store
the tunnel step provided by the output port
3. Determine which port (either input or output) is the buffer
supplier, and call OMX_SetParameter on the output port to
indicate this selection.
The component will return from this call within 5 msec.
@param [in] hComp
Handle of the component to be accessed. This is the component
handle returned by the call to the OMX_GetHandle method.
@param [in] nPort
nPort is used to select the port on the component to be used
for tunneling.
@param [in] hTunneledComp
Handle of the component to tunnel with. This is the component
handle returned by the call to the OMX_GetHandle method. When
this parameter is 0x0 the component should setup the port for
communication with the application / IL Client.
@param [in] nPortOutput
nPortOutput is used indicate the port the component should
tunnel with.
@param [in] pTunnelSetup
Pointer to the tunnel setup structure. When nPort is an output port
the component should populate the fields of this structure. When
When nPort is an input port the component should review the setup
provided by the component with the output port.
@return OMX_ERRORTYPE
If the command successfully executes, the return code will be
OMX_ErrorNone. Otherwise the appropriate OMX error will be returned.
@ingroup tun
*/
OMX_ERRORTYPE (*ComponentTunnelRequest)(
OMX_IN OMX_HANDLETYPE hComp,
OMX_IN OMX_U32 nPort,
OMX_IN OMX_HANDLETYPE hTunneledComp,
OMX_IN OMX_U32 nTunneledPort,
OMX_INOUT OMX_TUNNELSETUPTYPE* pTunnelSetup);
/** refer to OMX_UseBuffer in OMX_core.h or the OMX IL
specification for details on the UseBuffer method.
@ingroup buf
*/
OMX_ERRORTYPE (*UseBuffer)(
OMX_IN OMX_HANDLETYPE hComponent,
OMX_INOUT OMX_BUFFERHEADERTYPE** ppBufferHdr,
OMX_IN OMX_U32 nPortIndex,
OMX_IN OMX_PTR pAppPrivate,
OMX_IN OMX_U32 nSizeBytes,
OMX_IN OMX_U8* pBuffer);
/** refer to OMX_AllocateBuffer in OMX_core.h or the OMX IL
specification for details on the AllocateBuffer method.
@ingroup buf
*/
OMX_ERRORTYPE (*AllocateBuffer)(
OMX_IN OMX_HANDLETYPE hComponent,
OMX_INOUT OMX_BUFFERHEADERTYPE** ppBuffer,
OMX_IN OMX_U32 nPortIndex,
OMX_IN OMX_PTR pAppPrivate,
OMX_IN OMX_U32 nSizeBytes);
/** refer to OMX_FreeBuffer in OMX_core.h or the OMX IL
specification for details on the FreeBuffer method.
@ingroup buf
*/
OMX_ERRORTYPE (*FreeBuffer)(
OMX_IN OMX_HANDLETYPE hComponent,
OMX_IN OMX_U32 nPortIndex,
OMX_IN OMX_BUFFERHEADERTYPE* pBuffer);
/** refer to OMX_EmptyThisBuffer in OMX_core.h or the OMX IL
specification for details on the EmptyThisBuffer method.
@ingroup buf
*/
OMX_ERRORTYPE (*EmptyThisBuffer)(
OMX_IN OMX_HANDLETYPE hComponent,
OMX_IN OMX_BUFFERHEADERTYPE* pBuffer);
/** refer to OMX_FillThisBuffer in OMX_core.h or the OMX IL
specification for details on the FillThisBuffer method.
@ingroup buf
*/
OMX_ERRORTYPE (*FillThisBuffer)(
OMX_IN OMX_HANDLETYPE hComponent,
OMX_IN OMX_BUFFERHEADERTYPE* pBuffer);
/** The SetCallbacks method is used by the core to specify the callback
structure from the application to the component. This is a blocking
call. The component will return from this call within 5 msec.
@param [in] hComponent
Handle of the component to be accessed. This is the component
handle returned by the call to the GetHandle function.
@param [in] pCallbacks
pointer to an OMX_CALLBACKTYPE structure used to provide the
callback information to the component
@param [in] pAppData
pointer to an application defined value. It is anticipated that
the application will pass a pointer to a data structure or a "this
pointer" in this area to allow the callback (in the application)
to determine the context of the call
@return OMX_ERRORTYPE
If the command successfully executes, the return code will be
OMX_ErrorNone. Otherwise the appropriate OMX error will be returned.
*/
OMX_ERRORTYPE (*SetCallbacks)(
OMX_IN OMX_HANDLETYPE hComponent,
OMX_IN OMX_CALLBACKTYPE* pCallbacks,
OMX_IN OMX_PTR pAppData);
/** ComponentDeInit method is used to deinitialize the component
providing a means to free any resources allocated at component
initialization. NOTE: After this call the component handle is
not valid for further use.
@param [in] hComponent
Handle of the component to be accessed. This is the component
handle returned by the call to the GetHandle function.
@return OMX_ERRORTYPE
If the command successfully executes, the return code will be
OMX_ErrorNone. Otherwise the appropriate OMX error will be returned.
*/
OMX_ERRORTYPE (*ComponentDeInit)(
OMX_IN OMX_HANDLETYPE hComponent);
/** @ingroup buf */
OMX_ERRORTYPE (*UseEGLImage)(
OMX_IN OMX_HANDLETYPE hComponent,
OMX_INOUT OMX_BUFFERHEADERTYPE** ppBufferHdr,
OMX_IN OMX_U32 nPortIndex,
OMX_IN OMX_PTR pAppPrivate,
OMX_IN void* eglImage);
OMX_ERRORTYPE (*ComponentRoleEnum)(
OMX_IN OMX_HANDLETYPE hComponent,
OMX_OUT OMX_U8 *cRole,
OMX_IN OMX_U32 nIndex);
} OMX_COMPONENTTYPE;
这个结构体主要有一些函数指针，当makeComponentInstance函数返回后，这些函数指针也已经赋值。
OMX_CALLBACKTYPE *callbacks，这个参数是一些回调函数，这些回调函数涉及了Codec通知上层传为decode的数据下来，解码完数据后，也是通过这些回调函数通知上层。
2）destroyComponentInstance，
virtual OMX_ERRORTYPE destroyComponentInstance(
OMX_COMPONENTTYPE *component) = 0;
销毁相应的Codec component。
3）enumerateComponents
virtual OMX_ERRORTYPE enumerateComponents(
OMX_STRING name,
size_t size,
OMX_U32 index) = 0;
通过index枚举出Components的名称name，
4）getRolesOfComponent
virtual OMX_ERRORTYPE getRolesOfComponent(
const char *name,
Vector<String8> *roles) = 0;
通过component的名称得知该Component是属于何种角色，encode还是decode。
Summarize
Omx提出的框架比较简单，把Codec的对接全部限定在OMXPluginBase这个抽象类规定的接口内，厂商通过继承实现该接口，把具体的Encode\Decode封装成，OMXPluginBase成为了Android MediaPlayer与Vender厂商具体实现之间的规则。厂商如何把不同的Codec实现，不受非常强烈的限制，但必须实现OMXPluginBase接口。
