ALSA: Add a documentation for channel mapping API
Signed-off-by: Takashi Iwai <tiwai@suse.de>
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Takashi Iwai
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Documentation/sound/alsa/Channel-Mapping-API.txt
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Documentation/sound/alsa/Channel-Mapping-API.txt
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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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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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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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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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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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corresponds to which (speaker) position. Thus, applications applied
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wrong channels for 5.1 outputs, and you hear suddenly strange sound
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from rear. Or, some devices secretly assume that center/LFE is the
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third/fourth channels while others that C/LFE as 5th/6th channels.
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Also, some devices such as HDMI are configurable for different speaker
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positions even with the same number of total channels. However, there
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was no way to specify this because of lack of channel map
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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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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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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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Note the name is different depending on the PCM substream direction.
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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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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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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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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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RR.
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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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here is a cut:
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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_FL, /* front left */
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SNDRV_CHMAP_FC, /* front center */
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SNDRV_CHMAP_FR, /* front right */
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SNDRV_CHMAP_FLC, /* front left center */
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SNDRV_CHMAP_FRC, /* front right center */
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SNDRV_CHMAP_RL, /* rear left */
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SNDRV_CHMAP_RC, /* rear center */
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SNDRV_CHMAP_RR, /* rear right */
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SNDRV_CHMAP_RLC, /* rear left center */
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SNDRV_CHMAP_RRC, /* rear right center */
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SNDRV_CHMAP_SL, /* side left */
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SNDRV_CHMAP_SR, /* side right */
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SNDRV_CHMAP_LFE, /* LFE */
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SNDRV_CHMAP_FLW, /* front left wide */
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SNDRV_CHMAP_FRW, /* front right wide */
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SNDRV_CHMAP_FLH, /* front left high */
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SNDRV_CHMAP_FCH, /* front center high */
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SNDRV_CHMAP_FRH, /* front right high */
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SNDRV_CHMAP_TC, /* top center */
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SNDRV_CHMAP_NA, /* N/A, silent */
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SNDRV_CHMAP_LAST = SNDRV_CHMAP_NA,
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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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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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SNDRV_CTL_TLVT_CHMAP_FIXED 16 NDRV_CHMAP_FL SNDRV_CHMAP_FR \
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SNDRV_CHMAP_RL SNDRV_CHMAP_RR
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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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#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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(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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don't follow the standard definition above but driver-specific.
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* READ OPERATION
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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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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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* WRITE OPERATION
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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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to pass an integer value containing the valid channel positions for
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all channels of the assigned PCM substream.
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This operation is allowed only at PCM PREPARED state. When called in
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other states, it shall return an error.
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