ALSA: compress offload API documentation

The patch adds the documentation file explaining the API
Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Signed-off-by: Vinod Koul <vinod.koul@linux.intel.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

Documentation/sound/alsa/compress_offload.txt

+		compress_offload.txt
+		=====================
+	Pierre-Louis.Bossart <pierre-louis.bossart@linux.intel.com>
+		Vinod Koul <vinod.koul@linux.intel.com>
+
+Overview
+
+Since its early days, the ALSA API was defined with PCM support or
+constant bitrates payloads such as IEC61937 in mind. Arguments and
+returned values in frames are the norm, making it a challenge to
+extend the existing API to compressed data streams.
+
+In recent years, audio digital signal processors (DSP) were integrated
+in system-on-chip designs, and DSPs are also integrated in audio
+codecs. Processing compressed data on such DSPs results in a dramatic
+reduction of power consumption compared to host-based
+processing. Support for such hardware has not been very good in Linux,
+mostly because of a lack of a generic API available in the mainline
+kernel.
+
+Rather than requiring a compability break with an API change of the
+ALSA PCM interface, a new 'Compressed Data' API is introduced to
+provide a control and data-streaming interface for audio DSPs.
+
+The design of this API was inspired by the 2-year experience with the
+Intel Moorestown SOC, with many corrections required to upstream the
+API in the mainline kernel instead of the staging tree and make it
+usable by others.
+
+Requirements
+
+The main requirements are:
+
+- separation between byte counts and time. Compressed formats may have
+  a header per file, per frame, or no header at all. The payload size
+  may vary from frame-to-frame. As a result, it is not possible to
+  estimate reliably the duration of audio buffers when handling
+  compressed data. Dedicated mechanisms are required to allow for
+  reliable audio-video synchronization, which requires precise
+  reporting of the number of samples rendered at any given time.
+
+- Handling of multiple formats. PCM data only requires a specification
+  of the sampling rate, number of channels and bits per sample. In
+  contrast, compressed data comes in a variety of formats. Audio DSPs
+  may also provide support for a limited number of audio encoders and
+  decoders embedded in firmware, or may support more choices through
+  dynamic download of libraries.
+
+- Focus on main formats. This API provides support for the most
+  popular formats used for audio and video capture and playback. It is
+  likely that as audio compression technology advances, new formats
+  will be added.
+
+- Handling of multiple configurations. Even for a given format like
+  AAC, some implementations may support AAC multichannel but HE-AAC
+  stereo. Likewise WMA10 level M3 may require too much memory and cpu
+  cycles. The new API needs to provide a generic way of listing these
+  formats.
+
+- Rendering/Grabbing only. This API does not provide any means of
+  hardware acceleration, where PCM samples are provided back to
+  user-space for additional processing. This API focuses instead on
+  streaming compressed data to a DSP, with the assumption that the
+  decoded samples are routed to a physical output or logical back-end.
+
+ - Complexity hiding. Existing user-space multimedia frameworks all
+  have existing enums/structures for each compressed format. This new
+  API assumes the existence of a platform-specific compatibility layer
+  to expose, translate and make use of the capabilities of the audio
+  DSP, eg. Android HAL or PulseAudio sinks. By construction, regular
+  applications are not supposed to make use of this API.
+
+
+Design
+
+The new API shares a number of concepts with with the PCM API for flow
+control. Start, pause, resume, drain and stop commands have the same
+semantics no matter what the content is.
+
+The concept of memory ring buffer divided in a set of fragments is
+borrowed from the ALSA PCM API. However, only sizes in bytes can be
+specified.
+
+Seeks/trick modes are assumed to be handled by the host.
+
+The notion of rewinds/forwards is not supported. Data committed to the
+ring buffer cannot be invalidated, except when dropping all buffers.
+
+The Compressed Data API does not make any assumptions on how the data
+is transmitted to the audio DSP. DMA transfers from main memory to an
+embedded audio cluster or to a SPI interface for external DSPs are
+possible. As in the ALSA PCM case, a core set of routines is exposed;
+each driver implementer will have to write support for a set of
+mandatory routines and possibly make use of optional ones.
+
+The main additions are
+
+- get_caps
+This routine returns the list of audio formats supported. Querying the
+codecs on a capture stream will return encoders, decoders will be
+listed for playback streams.
+
+- get_codec_caps For each codec, this routine returns a list of
+capabilities. The intent is to make sure all the capabilities
+correspond to valid settings, and to minimize the risks of
+configuration failures. For example, for a complex codec such as AAC,
+the number of channels supported may depend on a specific profile. If
+the capabilities were exposed with a single descriptor, it may happen
+that a specific combination of profiles/channels/formats may not be
+supported. Likewise, embedded DSPs have limited memory and cpu cycles,
+it is likely that some implementations make the list of capabilities
+dynamic and dependent on existing workloads. In addition to codec
+settings, this routine returns the minimum buffer size handled by the
+implementation. This information can be a function of the DMA buffer
+sizes, the number of bytes required to synchronize, etc, and can be
+used by userspace to define how much needs to be written in the ring
+buffer before playback can start.
+
+- set_params
+This routine sets the configuration chosen for a specific codec. The
+most important field in the parameters is the codec type; in most
+cases decoders will ignore other fields, while encoders will strictly
+comply to the settings
+
+- get_params
+This routines returns the actual settings used by the DSP. Changes to
+the settings should remain the exception.
+
+- get_timestamp
+The timestamp becomes a multiple field structure. It lists the number
+of bytes transferred, the number of samples processed and the number
+of samples rendered/grabbed. All these values can be used to determine
+the avarage bitrate, figure out if the ring buffer needs to be
+refilled or the delay due to decoding/encoding/io on the DSP.
+
+Note that the list of codecs/profiles/modes was derived from the
+OpenMAX AL specification instead of reinventing the wheel.
+Modifications include:
+- Addition of FLAC and IEC formats
+- Merge of encoder/decoder capabilities
+- Profiles/modes listed as bitmasks to make descriptors more compact
+- Addition of set_params for decoders (missing in OpenMAX AL)
+- Addition of AMR/AMR-WB encoding modes (missing in OpenMAX AL)
+- Addition of format information for WMA
+- Addition of encoding options when required (derived from OpenMAX IL)
+- Addition of rateControlSupported (missing in OpenMAX AL)
+
+Not supported:
+
+- Support for VoIP/circuit-switched calls is not the target of this
+  API. Support for dynamic bit-rate changes would require a tight
+  coupling between the DSP and the host stack, limiting power savings.
+
+- Packet-loss concealment is not supported. This would require an
+  additional interface to let the decoder synthesize data when frames
+  are lost during transmission. This may be added in the future.
+
+- Volume control/routing is not handled by this API. Devices exposing a
+  compressed data interface will be considered as regular ALSA devices;
+  volume changes and routing information will be provided with regular
+  ALSA kcontrols.
+
+- Embedded audio effects. Such effects should be enabled in the same
+  manner, no matter if the input was PCM or compressed.
+
+- multichannel IEC encoding. Unclear if this is required.
+
+- Encoding/decoding acceleration is not supported as mentioned
+  above. It is possible to route the output of a decoder to a capture
+  stream, or even implement transcoding capabilities. This routing
+  would be enabled with ALSA kcontrols.
+
+- Audio policy/resource management. This API does not provide any
+  hooks to query the utilization of the audio DSP, nor any premption
+  mechanisms.
+
+- No notion of underun/overrun. Since the bytes written are compressed
+  in nature and data written/read doesn't translate directly to
+  rendered output in time, this does not deal with underrun/overun and
+  maybe dealt in user-library
+
+Credits:
+- Mark Brown and Liam Girdwood for discussions on the need for this API
+- Harsha Priya for her work on intel_sst compressed API
+- Rakesh Ughreja for valuable feedback
+- Sing Nallasellan, Sikkandar Madar and Prasanna Samaga for
+  demonstrating and quantifying the benefits of audio offload on a
+  real platform.