ALSA: Add a documentation for channel mapping API

Signed-off-by: NTakashi Iwai <tiwai@suse.de>

Documentation/sound/alsa/Channel-Mapping-API.txt

+ALSA PCM channel-mapping API
+============================
+					Takashi Iwai <tiwai@suse.de>
+
+GENERAL
+-------
+
+The channel mapping API allows user to query the possible channel maps
+and the current channel map, also optionally to modify the channel map
+of the current stream.
+
+A channel map is an array of position for each PCM channel.
+Typically, a stereo PCM stream has a channel map of
+  { front_left, front_right }
+while a 4.0 surround PCM stream has a channel map of
+  { front left, front right, rear left, rear right }.
+
+The problem, so far, was that we had no standard channel map
+explicitly, and applications had no way to know which channel
+corresponds to which (speaker) position.  Thus, applications applied
+wrong channels for 5.1 outputs, and you hear suddenly strange sound
+from rear.  Or, some devices secretly assume that center/LFE is the
+third/fourth channels while others that C/LFE as 5th/6th channels.
+
+Also, some devices such as HDMI are configurable for different speaker
+positions even with the same number of total channels.  However, there
+was no way to specify this because of lack of channel map
+specification.  These are the main motivations for the new channel
+mapping API.
+
+
+DESIGN
+------
+
+Actually, "the channel mapping API" doesn't introduce anything new in
+the kernel/user-space ABI perspective.  It uses only the existing
+control element features.
+
+As a ground design, each PCM substream may contain a control element
+providing the channel mapping information and configuration.  This
+element is specified by:
+	iface = SNDRV_CTL_ELEM_IFACE_PCM
+	name = "Playback Channel Map" or "Capture Channel Map"
+	device = the same device number for the assigned PCM substream
+	index = the same index number for the assigned PCM substream
+
+Note the name is different depending on the PCM substream direction.
+
+Each control element provides at least the TLV read operation and the
+read operation.  Optionally, the write operation can be provided to
+allow user to change the channel map dynamically.
+
+* TLV
+
+The TLV operation gives the list of available channel
+maps.  A list item of a channel map is usually a TLV of
+	type data-bytes ch0 ch1 ch2...
+where type is the TLV type value, the second argument is the total
+bytes (not the numbers) of channel values, and the rest are the
+position value for each channel.
+
+As a TLV type, either SNDRV_CTL_TLVT_CHMAP_FIXED,
+SNDRV_CTL_TLV_CHMAP_VAR or SNDRV_CTL_TLVT_CHMAP_PAIRED can be used.
+The _FIXED type is for a channel map with the fixed channel position
+while the latter two are for flexible channel positions.  _VAR type is
+for a channel map where all channels are freely swappable and _PAIRED
+type is where pair-wise channels are swappable.  For example, when you
+have {FL/FR/RL/RR} channel map, _PAIRED type would allow you to swap
+only {RL/RR/FL/FR} while _VAR type would allow even swapping FL and
+RR.
+
+These new TLV types are defined in sound/tlv.h.
+
+The available channel position values are defined in sound/asound.h,
+here is a cut:
+
+/* channel positions */
+enum {
+	SNDRV_CHMAP_UNKNOWN = 0,
+	SNDRV_CHMAP_FL,		/* front left */
+	SNDRV_CHMAP_FC,		/* front center */
+	SNDRV_CHMAP_FR,		/* front right */
+	SNDRV_CHMAP_FLC,	/* front left center */
+	SNDRV_CHMAP_FRC,	/* front right center */
+	SNDRV_CHMAP_RL,		/* rear left */
+	SNDRV_CHMAP_RC,		/* rear center */
+	SNDRV_CHMAP_RR,		/* rear right */
+	SNDRV_CHMAP_RLC,	/* rear left center */
+	SNDRV_CHMAP_RRC,	/* rear right center */
+	SNDRV_CHMAP_SL,		/* side left */
+	SNDRV_CHMAP_SR,		/* side right */
+	SNDRV_CHMAP_LFE,	/* LFE */
+	SNDRV_CHMAP_FLW,	/* front left wide */
+	SNDRV_CHMAP_FRW,	/* front right wide */
+	SNDRV_CHMAP_FLH,	/* front left high */
+	SNDRV_CHMAP_FCH,	/* front center high */
+	SNDRV_CHMAP_FRH,	/* front right high */
+	SNDRV_CHMAP_TC,		/* top center */
+	SNDRV_CHMAP_NA,		/* N/A, silent */
+	SNDRV_CHMAP_LAST = SNDRV_CHMAP_NA,
+};
+
+When a PCM stream can provide more than one channel map, you can
+provide multiple channel maps in a TLV container type.  The TLV data
+to be returned will contain such as:
+	SNDRV_CTL_TLVT_CONTAINER 96
+	    SNDRV_CTL_TLVT_CHMAP_FIXED 4 SNDRV_CHMAP_FC
+	    SNDRV_CTL_TLVT_CHMAP_FIXED 8 SNDRV_CHMAP_FL SNDRV_CHMAP_FR
+	    SNDRV_CTL_TLVT_CHMAP_FIXED 16 NDRV_CHMAP_FL SNDRV_CHMAP_FR \
+		SNDRV_CHMAP_RL SNDRV_CHMAP_RR
+
+The channel position is provided in LSB 16bits.  The upper bits are
+used for bit flags.
+
+#define SNDRV_CHMAP_POSITION_MASK	0xffff
+#define SNDRV_CHMAP_PHASE_INVERSE	(0x01 << 16)
+#define SNDRV_CHMAP_DRIVER_SPEC		(0x02 << 16)
+
+SNDRV_CHMAP_PHASE_INVERSE indicates the channel is phase inverted,
+(thus summing left and right channels would result in almost silence).
+Some digital mic devices have this.
+
+When SNDRV_CHMAP_DRIVER_SPEC is set, all the channel position values
+don't follow the standard definition above but driver-specific.
+
+* READ OPERATION
+
+The control read operation is for providing the current channel map of
+the given stream.  The control element returns an integer array
+containing the position of each channel.
+
+When this is performed before the number of the channel is specified
+(i.e. hw_params is set), it should return all channels set to
+UNKNOWN.
+
+* WRITE OPERATION
+
+The control write operation is optional, and only for devices that can
+change the channel configuration on the fly, such as HDMI.  User needs
+to pass an integer value containing the valid channel positions for
+all channels of the assigned PCM substream.
+
+This operation is allowed only at PCM PREPARED state.  When called in
+other states, it shall return an error.