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ALSA: doc: ReSTize Channel-Mapping-API document
A simple conversion from a text file. Put to designs subdirectory. Signed-off-by: Takashi Iwai <tiwai@suse.de>
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Takashi Iwai
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@ -1,9 +1,11 @@
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============================
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ALSA PCM channel-mapping API
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============================
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Takashi Iwai <tiwai@suse.de>
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GENERAL
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-------
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Takashi Iwai <tiwai@suse.de>
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General
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=======
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The channel mapping API allows user to query the possible channel maps
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and the current channel map, also optionally to modify the channel map
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@ -11,9 +13,9 @@ of the current stream.
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A channel map is an array of position for each PCM channel.
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Typically, a stereo PCM stream has a channel map of
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{ front_left, front_right }
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``{ front_left, front_right }``
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while a 4.0 surround PCM stream has a channel map of
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{ front left, front right, rear left, rear right }.
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``{ front left, front right, rear left, rear right }.``
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The problem, so far, was that we had no standard channel map
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explicitly, and applications had no way to know which channel
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@ -29,8 +31,8 @@ specification. These are the main motivations for the new channel
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mapping API.
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DESIGN
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------
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Design
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======
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Actually, "the channel mapping API" doesn't introduce anything new in
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the kernel/user-space ABI perspective. It uses only the existing
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@ -39,10 +41,11 @@ control element features.
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As a ground design, each PCM substream may contain a control element
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providing the channel mapping information and configuration. This
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element is specified by:
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iface = SNDRV_CTL_ELEM_IFACE_PCM
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name = "Playback Channel Map" or "Capture Channel Map"
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device = the same device number for the assigned PCM substream
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index = the same index number for the assigned PCM substream
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* iface = SNDRV_CTL_ELEM_IFACE_PCM
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* name = "Playback Channel Map" or "Capture Channel Map"
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* device = the same device number for the assigned PCM substream
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* index = the same index number for the assigned PCM substream
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Note the name is different depending on the PCM substream direction.
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@ -50,32 +53,35 @@ Each control element provides at least the TLV read operation and the
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read operation. Optionally, the write operation can be provided to
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allow user to change the channel map dynamically.
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* TLV
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TLV
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---
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The TLV operation gives the list of available channel
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maps. A list item of a channel map is usually a TLV of
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type data-bytes ch0 ch1 ch2...
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``type data-bytes ch0 ch1 ch2...``
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where type is the TLV type value, the second argument is the total
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bytes (not the numbers) of channel values, and the rest are the
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position value for each channel.
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As a TLV type, either SNDRV_CTL_TLVT_CHMAP_FIXED,
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SNDRV_CTL_TLV_CHMAP_VAR or SNDRV_CTL_TLVT_CHMAP_PAIRED can be used.
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The _FIXED type is for a channel map with the fixed channel position
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while the latter two are for flexible channel positions. _VAR type is
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for a channel map where all channels are freely swappable and _PAIRED
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As a TLV type, either ``SNDRV_CTL_TLVT_CHMAP_FIXED``,
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``SNDRV_CTL_TLV_CHMAP_VAR`` or ``SNDRV_CTL_TLVT_CHMAP_PAIRED`` can be used.
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The ``_FIXED`` type is for a channel map with the fixed channel position
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while the latter two are for flexible channel positions. ``_VAR`` type is
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for a channel map where all channels are freely swappable and ``_PAIRED``
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type is where pair-wise channels are swappable. For example, when you
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have {FL/FR/RL/RR} channel map, _PAIRED type would allow you to swap
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only {RL/RR/FL/FR} while _VAR type would allow even swapping FL and
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have {FL/FR/RL/RR} channel map, ``_PAIRED`` type would allow you to swap
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only {RL/RR/FL/FR} while ``_VAR`` type would allow even swapping FL and
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RR.
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These new TLV types are defined in sound/tlv.h.
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These new TLV types are defined in ``sound/tlv.h``.
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The available channel position values are defined in sound/asound.h,
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The available channel position values are defined in ``sound/asound.h``,
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here is a cut:
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/* channel positions */
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enum {
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::
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/* channel positions */
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enum {
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SNDRV_CHMAP_UNKNOWN = 0,
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SNDRV_CHMAP_NA, /* N/A, silent */
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SNDRV_CHMAP_MONO, /* mono stream */
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@ -107,11 +113,13 @@ enum {
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SNDRV_CHMAP_TRR, /* top rear right */
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SNDRV_CHMAP_TRC, /* top rear center */
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SNDRV_CHMAP_LAST = SNDRV_CHMAP_TRC,
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};
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};
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When a PCM stream can provide more than one channel map, you can
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provide multiple channel maps in a TLV container type. The TLV data
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to be returned will contain such as:
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::
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SNDRV_CTL_TLVT_CONTAINER 96
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SNDRV_CTL_TLVT_CHMAP_FIXED 4 SNDRV_CHMAP_FC
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SNDRV_CTL_TLVT_CHMAP_FIXED 8 SNDRV_CHMAP_FL SNDRV_CHMAP_FR
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@ -120,19 +128,21 @@ to be returned will contain such as:
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The channel position is provided in LSB 16bits. The upper bits are
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used for bit flags.
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::
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#define SNDRV_CHMAP_POSITION_MASK 0xffff
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#define SNDRV_CHMAP_PHASE_INVERSE (0x01 << 16)
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#define SNDRV_CHMAP_DRIVER_SPEC (0x02 << 16)
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#define SNDRV_CHMAP_POSITION_MASK 0xffff
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#define SNDRV_CHMAP_PHASE_INVERSE (0x01 << 16)
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#define SNDRV_CHMAP_DRIVER_SPEC (0x02 << 16)
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SNDRV_CHMAP_PHASE_INVERSE indicates the channel is phase inverted,
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``SNDRV_CHMAP_PHASE_INVERSE`` indicates the channel is phase inverted,
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(thus summing left and right channels would result in almost silence).
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Some digital mic devices have this.
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When SNDRV_CHMAP_DRIVER_SPEC is set, all the channel position values
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When ``SNDRV_CHMAP_DRIVER_SPEC`` is set, all the channel position values
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don't follow the standard definition above but driver-specific.
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* READ OPERATION
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Read Operation
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--------------
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The control read operation is for providing the current channel map of
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the given stream. The control element returns an integer array
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@ -140,9 +150,10 @@ containing the position of each channel.
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When this is performed before the number of the channel is specified
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(i.e. hw_params is set), it should return all channels set to
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UNKNOWN.
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``UNKNOWN``.
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* WRITE OPERATION
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Write Operation
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---------------
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The control write operation is optional, and only for devices that can
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change the channel configuration on the fly, such as HDMI. User needs
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@ -4,5 +4,6 @@ Designs and Implementations
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.. toctree::
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:maxdepth: 2
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channel-mapping-api
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procfile
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powersave
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