Commit de08e701 authored by Mauro Carvalho Chehab's avatar Mauro Carvalho Chehab

[media] DocBook: Add documentation about the demux API

There are several stuff at media's kdapi.xml that don't
belong there, as it documents the Kernel internal ABI, and
not the userspace API.

Add the documentation here. The hole kdapi.xml will be
removed on a latter patch, after we finish documenting
what's there at the proper places.
Signed-off-by: default avatarMauro Carvalho Chehab <>
parent 9be91db2
......@@ -239,21 +239,86 @@ X!Isound/sound_firmware.c
<sect1><title>Digital TV (DVB) devices</title>
<sect1><title>Remote Controller devices</title>
<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
<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
<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.
<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
&#8220;bottom half&#8221; mechanism or start a tasklet instead of
making the callback function call directly from a hardware
<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>
<sect1><title>Remote Controller devices</title>
<sect1><title>Media Controller devices</title>
<sect1><title>Media Controller devices</title>
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