[media] DocBook: Add documentation about the demux API

     </sect1>

     <sect1><title>Digital TV (DVB) devices</title>

!Idrivers/media/dvb-core/dvb_ca_en50221.h

!Idrivers/media/dvb-core/demux.h

!Idrivers/media/dvb-core/dvb_frontend.h

!Idrivers/media/dvb-core/dvb_math.h

!Idrivers/media/dvb-core/dvb_ringbuffer.h

!Idrivers/media/dvb-core/dvbdev.h

	<sect1><title>Digital TV Demux API</title>

	    <para>The kernel demux API defines a driver-internal interface for

	    registering low-level, hardware specific driver to a hardware

	    independent demux layer. It is only of interest for Digital TV

	    device driver writers. The header file for this API is named

	    <constant>demux.h</constant> and located in

	    <constant>drivers/media/dvb-core</constant>.</para>

	<para>The demux API should be implemented for each demux in the

	system. It is used to select the TS source of a demux and to manage

	the demux resources. When the demux client allocates a resource via

	the demux API, it receives a pointer to the API of that

	resource.</para>

	<para>Each demux receives its TS input from a DVB front-end or from

	memory, as set via this demux API. In a system with more than one

	front-end, the API can be used to select one of the DVB front-ends

	as a TS source for a demux, unless this is fixed in the HW platform.

	The demux API only controls front-ends regarding to their connections

	with demuxes; the APIs used to set the other front-end parameters,

	such as tuning, are not defined in this document.</para>

	<para>The functions that implement the abstract interface demux should

	be defined static or module private and registered to the Demux

	core for external access. It is not necessary to implement every

	function in the struct <constant>dmx_demux</constant>. For example,

	a demux interface might support Section filtering, but not PES

	filtering. The API client is expected to check the value of any

	function pointer before calling the function: the value of NULL means

	that the &#8220;function is not available&#8221;.</para>

	<para>Whenever the functions of the demux API modify shared data,

	the possibilities of lost update and race condition problems should

	be addressed, e.g. by protecting parts of code with mutexes.</para>

	<para>Note that functions called from a bottom half context must not

	sleep. Even a simple memory allocation without using GFP_ATOMIC can

	result in a kernel thread being put to sleep if swapping is needed.

	For example, the Linux kernel calls the functions of a network device

	interface from a bottom half context. Thus, if a demux API function

	is called from network device code, the function must not sleep.

	</para>

    </sect1>

    <section id="demux_callback_api">

	<title>Demux Callback API</title>

	<para>This kernel-space API comprises the callback functions that

	deliver filtered data to the demux client. Unlike the other DVB

	kABIs, these functions are provided by the client and called from

	the demux code.</para>

	<para>The function pointers of this abstract interface are not

	packed into a structure as in the other demux APIs, because the

	callback functions are registered and used independent of each

	other. As an example, it is possible for the API client to provide

	several callback functions for receiving TS packets and no

	callbacks for PES packets or sections.</para>

	<para>The functions that implement the callback API need not be

	re-entrant: when a demux driver calls one of these functions,

	the driver is not allowed to call the function again before

	the original call returns. If a callback is triggered by a

	hardware interrupt, it is recommended to use the Linux

	“bottom half” mechanism or start a tasklet instead of

	making the callback function call directly from a hardware

	interrupt.</para>

	<para>This mechanism is implemented by

	<link linkend='API-dmx-ts-cb'>dmx_ts_cb()</link> and

	<link linkend='API-dmx-section-cb'>dmx_section_cb()</link>.</para>

    </section>

!Idrivers/media/dvb-core/demux.h

    </sect1>

    <sect1><title>Remote Controller devices</title>

!Iinclude/media/rc-core.h