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+This document gives an overview of TDM framework and its interface with low
+level drivers and upper level users/clients.
+
+Terminology:
+============
+1. Adapter or TDM adapter: Refers to an instance of TDM controller/device on
+ the system.
+2. TDM channel: The channel is the smallest entity on which all the TDM
+ read/write operations will occur. Technically all channels map to a set of
+ consecutive time slots on the physical TDM frame. The channels will be
+ dynamically created and destroyed using tdm_open_channel and
+ tdm_close_channel.
+3. TDM frame: Is a set of TDM channels which is transmitted sequentially over
+ time. The frame start is identified by a frame sync signal that is briefly
+ asserted at the beginning of each frame.
+
+<----------TDM Frame 0------------>|<-----TDM Frame 1----------------->
+
+|----|----|----|----|----|----|----|----|----|----|----|----|----|----|
+| 0 | 1 | 2 | 3 | 4 | ...| n | 0 | 1 | 2 | 3 | 4 | ...| n |
+|----|----|----|----|----|----|----|----|----|----|----|----|----|----|
+<----> <---->
+ ch 0 ch 0
+
+4. TDM client: Application/driver which registers with TDM framework to use TDM
+ device.
+
+TDM modes:
+==========
+A TDM device can operate in one of the following modes:
+1. Single port full mode - Single user, no interleaving
+2. Single port channelised mode (raw, E1, T1)- many users using different channels
+3. Single port fractional mode -
+4. Multi port mode - multiple users using different ports in different configurations.
+
+All the above configurations differ in number of TDM client they support,
+number of TDM channels and number of TDM ports.
+
+Currently we are supporting only single port channelised mode. Hence all the explanations below
+refer to channelised mode of TDM. This framework can be easily extended to support other modes.
+
+Single port Channelised Mode:
+=============================
+In single port channelised mode there can be only one port and each channel can have only one time slot.
+The number of active channels can be less than the maximum supported channels/slots.
+
+<----------TDM Frame 0------------>|<-----TDM Frame 1----------------->
+
+|----|----|----|----|----|----|----|----|----|----|----|----|----|----|
+| 0 | 1 | 2 | 3 | 4 | ...| n | 0 | 1 | 2 | 3 | 4 | ...| n |...
+|----|----|----|----|----|----|----|----|----|----|----|----|----|----|
+<----><---> <---->
+ ch 0 ch1 ch 0
+client0 client1
+
+TDM Subsystem Overview:
+=======================
+
+-------------------------------------------------------------------
+ |------------------------|
+ | kernel mode TDM clients|
+ |------------------------|
+ ||
+ ||
+ client register
+ ||
+ ||
+ \/
+ ______________________________________________________________
+ | |
+ | client interface |
+ |------------------------------------------------------------|
+ | TDM Subsystem Framework |
+ | (tdm-core.c) |
+ | |
+ | ->buffer handling |
+ | ->interleaving/de-interleaving |
+ | |
+ |------------------------------------------------------------|
+ | TDM interface Line control interface |
+ |____________________________________________________________|
+ /\ /\
+ || ||
+ device register device register
+ || ||
+ || ||
+
+ fsl_tdm.c ucc_tdm.c slic_zarlink.c framer.c
+--------------------------------------------------------------------------
+_______________________ _____________________ ________ ________
+| | | | | | | |
+|[h/w] TDM controller | |UCC TDM controller | | SLIC | |Framer|
+|_____________________| |___________________| |______| |______|
+
+
+TDM Adapter Registration:
+=========================
+All the TDM adapter drivers will get registered as platform drivers to Linux.
+For every instance of the TDM adapter the relevant driver will be probed.
+As part of probe the driver will
+1. Do the basic initialization in terms of memory allocation for various
+ driver specific data structures, interrupts registration, etc.
+2. Initialize the TDM adapter and configure it in default configuration.
+ like operating mode, number of channels, channel type, etc.
+3. Add the TDM adapter to the TDM Framework via tdm_add_adapter() API.
+ As a result TDM framework will add this adapter to it's queue of
+ available adapters. As part of this adapter registration TDM framework
+ is also supplied a list of access algorithms for the particular TDM
+ adapter.
+4. Notifies the clients
+
+TDM Client Registration:
+========================
+Every TDM client gets itself registered with the TDM framework layer as
+a TDM driver using the API tdm_add_driver(). As part of this the TDM client
+supplies to the TDM framework the adapter with which it wants to hook and
+the function pointers of attach and detach functions which must be called
+as soon as the requested adapter is available.
+
+As a result the TDM framework keeps association of TDM adapters and TDM
+client drivers.
+As soon as this association gets established a tasklet is created for the
+adapter which is handled by tdm_data_tasklet_fn. The primary function of
+this tasklet acts as an interface to transfer the TDM data between the
+TDM adapter and the TDM client drivers.
+
+Currently TDM adaper can only be used in the default configuration.
+ie the configuration cannot be modified by TDM clients. This will be
+enhanced in future.
+
+Line Control Device Registration:
+=================================
+Each line control device has to add itself to list of available tdm phy device
+list by calling tdm_add_phy. At this point the framework traverses the list of
+available tdm devices to check if this tdm phy belongs to an already
+registered tdm device. If a match is found, then this phy attaches itself to
+that tdm device. Similarly the list of available tdm phys is traversed when a
+new tdm device registers itself.
+
+Data handling:
+==============
+Some basic assumptions about data handling:
+
+1. As per standard voice rate of 8Khz or 8192Hz. Which means 8192 samples must
+be sent every second. So if there are multiple clients sending voice data
+over TDM interface the rate should be such that the individual samples
+sent by them must be transmitted at 8 KHz.
+
+This is defined in the driver as
+
+ #define CH_1_MS_FRAMES 8
+
+2. Number of milliseconds at which TDM Rx interrupts occur
+This is basically the time for which the TDM data is sent in one Tx or Rx
+cycle of TDM controller hardware. In one DMA we send the data for 10ms.
+This gives enough time so that no buffer overflow or under-run occurs for
+transmit and receive respectively.
+
+ #define NUM_MS 10
+
+3. TDM has programmable slot length (8 bits or 16 bits). This can be configured by:
+
+ #define CHANNEL_8BIT_LIN 0 /* 8 bit linear */
+ #define CHANNEL_8BIT_ULAW 1 /* 8 bit Mu-law */
+ #define CHANNEL_8BIT_ALAW 2 /* 8 bit A-law */
+ #define CHANNEL_16BIT_LIN 3 /* 16 bit Linear */
+
+depending on the type of sample. For example the sample could be 16 bit linear
+or 8bit u-law encoded etc. Presently only word length of 16 is supported
+which is the default configuration.
+
+4. Number of channels means the total number of channels per TDM port.
+For example for E1 mode it will be 24, for T1 it will be 32, etc.
+There can also be raw mode, where the use case is not E1 or T1.
+Here the number of channels can be any number as per the use case.
+
+The whole framework follows a triple buffer approach to make sure that TDM data
+is played continuously at the desired rate.
+
+Buffers Involved:
+=================
+
+1.TDM driver or device buffers:
+These buffers are the device level buffers. They contain the TDM data
+which is transmitted/received on the TDM physical signals. As such these buffers must be
+allocated from driver layer so that all the hardware requirements are met.
+As an optimized design to remove extra memcopies, the client can
+pass the data in the same buffers. But this is only true for full mode of
+TDM. Where the user data can be straightaway passed to the hardware for
+transmission. Although in other cases memcopy cannot be avoided, because
+the framework layer will have to interleave the individual channels data to
+create the TDM frame data buffer. The size of this buffer will be governed by
+
+For channelised mode:
+The size of this buffer will be
+
+- number of channels
+- number of slots per channel
+- number of bytes per slot
+- number of frames per ms
+- number of ms
+
+For a channelised mode with single port the size of the device level buffer
+will be:
+channels * slots per channel * bytes per slot * frames per ms * number of ms
+channels * 1 * NUM_BYTES_PER_SLOT * NUM_MS * CH_1_MS_FRAMES
+
+There will be 3 such buffers.
+
+2.Channel level buffers:
+In case the TDM device is configured for multiport/multichannel the Framework
+layer needs to maintain the data for each channel. Hence for each channel
+opened a Buffer Descriptor ring of 3 BDs(see note below) is allocated both for
+transmit and receive. The client reads from/writes to the buffers pointed by
+these BD rings.
+
+The framework layer maintains a Data Process Tasklet per TDM
+device which is scheduled from every Rx interrupt. The interrupt handling
+periodicity is governed by the TDM data buffer size configured in the above
+section.
+The data tasklet when scheduled, will do Rx and Tx processing to copy
+the data from/to the channel specific interleaved buffers. The TDM
+controller will DMA the data which is copied in the interleaved buffers
+ or device level buffers.
+
+TDM framework provides the port level APIs and channel level APIs to the TDM
+client drivers to send and receive the respective data on different TDM slots.
+
+
+num of buffers = 3
+
+TDM client1 TDM Client2
+
+buf0------->buf1 buf0------->buf1
+^ | ^ |
+| V | V
+----buf2------ ------buf2----
+ | |
+ | |
+ | |
+ V V
+-----------------------------------------
+| |
+| DATA Tasklet |
+| |
+-----------------------------------------
+ |
+ |
+ V
+-----------------------------------------
+| TDM buffer interleaved * 3 |
+-----------------------------------------
+
+
+Not Implemented/Future work:
+============================
+1. TDM client will use the default configuration which is done at init time and
+ is not configurable. In future this should be made configurable as per the
+ need of client.
+2. The TDM framework still needs to be enhanced to configure the ports and their
+ attributes. Currently only single port channelised mode is supported.
+3. Line control interface is not available in the framework presently.
+ Presently it offer very minimal kind of interface.
+4. SLIC interface will be enhanced as per Zarlink Open source APIs in future.
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+Time Division Multiplexing (TDM)
+=================================
+
+TDM stands for Time Division Multiplexing. TDM is a type of digital or analog multiplexing in which two or more bit
+streams or signals are transferred apparently simultaneously as sub-channels
+in one communication channel, but are physically taking turns on the channel.
+
+The time domain is divided into several recurrent timeslots of fixed duration.
+These timeslot are grouped together to form a channel. A sample byte or data
+block of channel 1 is transmitted during timeslots allocated to channel 1,
+channel 2 during timeslot for channel 2, etc.
+
+One TDM frame consists of multiple channels. After the last channel the cycle
+starts all over again with a new frame, starting with the second sample, byte
+or data block from channel 1, and so on.
+
+X----------TDM Frame 0-------------X------TDM Frame 1-----------------X
+|----|----|----|----|----|----|----|----|----|----|----|----|----|----|
+| 0 | 1 | 2 | 3 | 4 | ...| n | 0 | 1 | 2 | 3 | 4 | ...| n |...
+|----|----|----|----|----|----|----|----|----|----|----|----|----|----|
+<----> <---->
+channel 0 channel 0
+-------------------------------------------------------------------->
+ Increasing Time
+
+Physical TDM interface
+=======================
+
+Physically TDM interface is a serial full duplex interface designed to
+communicate with variety of serial devices like industry standard framers,
+codecs, other DSPs, and microprocessors. It is typically used to transfer
+samples in a periodic manner. The TDM consists of independent transmitter and
+receiver sections with independent clock generation and frame synchronization.
+
+External TDM signals are:
+1. TDM_TCK: TDM Transmit clock
+2. TDM_RCK: TDM Receive clock
+3. TDM_TFS: TDM Tx frame sync to identify frame boundary
+4. TDM_RFS: TDM Rx Frame sync to identify frame boundary
+5. TDM_TD: TDM Tx data
+6. TDM_RD: TDM Rx data
+
+TDM is generally used to simultaneously transmit periodic data for multiple
+users. Common use cases would be to carry voice for various telephone
+subscribers in the telephone networks. It is widely used to carry telephonic
+data of various industry standards like E1/T1 data, etc.
+
+T1 Details
+==========
+T1 frame consists of 24 channels of 8 bits each plus one frame alignment bit.
+So a T1 frame has a total of 24x8 + 1 = 193 bits. Since each T1 frame contains
+one byte of voice data for each of 24 channels and the system needs to maintain
+a data rate of 8000 samples/sec. This would require 8000 frames/sec to be sent,
+yielding a data rate of 8000x193 bit/sec = 1.544 Mbps.
+
+E1 Details
+===========
+E1 frame consists of 32 channels each of 8 bits. Thus having a total frame
+length of 32x8 = 256 bits. Similar to the case of T1 it has to maintain a data
+rate of 8000 frames/sec. Thus having a data rate of 8000 x 256 bits/sec =
+2.048 Mbps.
+
+TDM use cases
+=============
+
+With SLIC kind of devices
+=========================
+SLIC stands for Subscriber Line Interface Card.
+Typically TDM systems consist of TDM controller and a line control device.
+
+The TDM controller interfaces to the line control device through TDM interface
+which is digital TDM multiplexed data.
+
+The Line controller has the functionality to interface with the TDM controller
+at one end and interface with the analog units at the other. For example if the
+line control device is a SLIC kind of device.
+The typical setup would be:
+
+|------------------|
+| |
+| | /-------\ |---------|
+| TDM controller |/ TDM \ | SLIC |<--------> s-ch0 analog phone 1
+| |\ data / | |<--------> s-ch1 analog phone 2
+| | \-------/ |---------|<--------> s-ch2 analog phone 3
+| |<----digital----> <analog>
+|------------------|
+
+
+
+Another use case (VoIP):
+========================
+
+ Voice packets on network
+ |--------| |------| _________ |------| |------|
+>----| |/---\| TDM | ( ) | TDM |/---\| |----->
+<----| SLIC |\---/| | ( n/w ) | |\---/| SLIC |-----<
+>----| | |------| --------- |------| | |----->
+ |--------| mux demux |------|
+
+In the above figure analog phones are connected to the hosts via SLICs.
+The voice spoken on the phones is multiplexed converted into VoIP packets
+and sent over network. At the rendering end the multiplexed data
+is de-multiplexed and sent to respective listeners via SLIC.