ALSA: doc: ReSTize compress-offload document

A simple conversion from a plain text file.
Put to designs subdirectory.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
This commit is contained in:
Takashi Iwai 2016-11-10 10:58:05 +01:00
parent 5a481fe309
commit e9df12c3ba
2 changed files with 66 additions and 54 deletions

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@ -1,10 +1,14 @@
compress_offload.txt
=====================
Pierre-Louis.Bossart <pierre-louis.bossart@linux.intel.com>
Vinod Koul <vinod.koul@linux.intel.com>
=========================
ALSA Compress-Offload API
=========================
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
@ -27,8 +31,9 @@ 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
Requirements
============
The main requirements are:
- separation between byte counts and time. Compressed formats may have
@ -63,7 +68,7 @@ The main requirements are:
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
- 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
@ -72,7 +77,7 @@ The main requirements are:
Design
======
The new API shares a number of concepts with the PCM API for flow
control. Start, pause, resume, drain and stop commands have the same
semantics no matter what the content is.
@ -95,43 +100,44 @@ 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_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.
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
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_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 average bitrate, figure out if the ring buffer needs to be
refilled or the delay due to decoding/encoding/io on the DSP.
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 average 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.
@ -145,6 +151,7 @@ Modifications include:
- Addition of encoding options when required (derived from OpenMAX IL)
- Addition of rateControlSupported (missing in OpenMAX AL)
Gapless Playback
================
When playing thru an album, the decoders have the ability to skip the encoder
@ -162,19 +169,19 @@ switch from one track to another and start using data for second track.
The main additions are:
- set_metadata
This routine sets the encoder delay and encoder padding. This can be used by
decoder to strip the silence. This needs to be set before the data in the track
is written.
set_metadata
This routine sets the encoder delay and encoder padding. This can be used by
decoder to strip the silence. This needs to be set before the data in the track
is written.
- set_next_track
This routine tells DSP that metadata and write operation sent after this would
correspond to subsequent track
set_next_track
This routine tells DSP that metadata and write operation sent after this would
correspond to subsequent track
- partial drain
This is called when end of file is reached. The userspace can inform DSP that
EOF is reached and now DSP can start skipping padding delay. Also next write
data would belong to next track
partial drain
This is called when end of file is reached. The userspace can inform DSP that
EOF is reached and now DSP can start skipping padding delay. Also next write
data would belong to next track
Sequence flow for gapless would be:
- Open
@ -189,10 +196,12 @@ Sequence flow for gapless would be:
- then call partial_drain to flush most of buffer in DSP
- Fill data of the next track
- DSP switches to second track
(note: order for partial_drain and write for next track can be reversed as well)
Not supported:
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.
@ -225,7 +234,9 @@ Not supported:
rendered output in time, this does not deal with underrun/overrun and
maybe dealt in user-library
Credits:
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

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@ -6,6 +6,7 @@ Designs and Implementations
control-names
channel-mapping-api
compress-offload
procfile
powersave
oss-emulation