85 lines
3.7 KiB
ReStructuredText
85 lines
3.7 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
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Digital TV Demux kABI
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---------------------
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Digital TV Demux
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~~~~~~~~~~~~~~~~
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The Kernel Digital TV Demux kABI defines a driver-internal interface for
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registering low-level, hardware specific driver to a hardware independent
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demux layer. It is only of interest for Digital TV device driver writers.
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The header file for this kABI is named ``demux.h`` and located in
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``include/media``.
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The demux kABI should be implemented for each demux in the system. It is
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used to select the TS source of a demux and to manage the demux resources.
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When the demux client allocates a resource via the demux kABI, it receives
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a pointer to the kABI of that resource.
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Each demux receives its TS input from a DVB front-end or from memory, as
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set via this demux kABI. In a system with more than one front-end, the kABI
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can be used to select one of the DVB front-ends as a TS source for a demux,
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unless this is fixed in the HW platform.
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The demux kABI only controls front-ends regarding to their connections with
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demuxes; the kABI used to set the other front-end parameters, such as
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tuning, are defined via the Digital TV Frontend kABI.
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The functions that implement the abstract interface demux should be defined
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static or module private and registered to the Demux core for external
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access. It is not necessary to implement every function in the struct
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:c:type:`dmx_demux`. For example, a demux interface might support Section filtering,
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but not PES filtering. The kABI client is expected to check the value of any
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function pointer before calling the function: the value of ``NULL`` means
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that the function is not available.
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Whenever the functions of the demux API modify shared data, the
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possibilities of lost update and race condition problems should be
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addressed, e.g. by protecting parts of code with mutexes.
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Note that functions called from a bottom half context must not sleep.
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Even a simple memory allocation without using ``GFP_ATOMIC`` can result in a
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kernel thread being put to sleep if swapping is needed. For example, the
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Linux Kernel calls the functions of a network device interface from a
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bottom half context. Thus, if a demux kABI function is called from network
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device code, the function must not sleep.
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Demux Callback API
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~~~~~~~~~~~~~~~~~~
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This kernel-space API comprises the callback functions that deliver filtered
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data to the demux client. Unlike the other DVB kABIs, these functions are
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provided by the client and called from the demux code.
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The function pointers of this abstract interface are not packed into a
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structure as in the other demux APIs, because the callback functions are
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registered and used independent of each other. As an example, it is possible
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for the API client to provide several callback functions for receiving TS
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packets and no callbacks for PES packets or sections.
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The functions that implement the callback API need not be re-entrant: when
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a demux driver calls one of these functions, the driver is not allowed to
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call the function again before the original call returns. If a callback is
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triggered by a hardware interrupt, it is recommended to use the Linux
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bottom half mechanism or start a tasklet instead of making the callback
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function call directly from a hardware interrupt.
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This mechanism is implemented by :c:func:`dmx_ts_cb()` and :c:func:`dmx_section_cb()`
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callbacks.
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Digital TV Demux device registration functions and data structures
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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.. kernel-doc:: include/media/dmxdev.h
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High-level Digital TV demux interface
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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.. kernel-doc:: include/media/dvb_demux.h
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Driver-internal low-level hardware specific driver demux interface
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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.. kernel-doc:: include/media/demux.h
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