+============================
ALSA PCM channel-mapping API
============================
- Takashi Iwai <tiwai@suse.de>
-GENERAL
--------
+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
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 }
+``{ front_left, front_right }``
while a 4.0 surround PCM stream has a channel map of
- { front left, front right, rear left, rear right }.
+``{ 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
mapping API.
-DESIGN
-------
+Design
+======
Actually, "the channel mapping API" doesn't introduce anything new in
the kernel/user-space ABI perspective. It uses only the existing
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
+
+* 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.
read operation. Optionally, the write operation can be provided to
allow user to change the channel map dynamically.
-* TLV
+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...
+``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
+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
+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.
+These new TLV types are defined in ``sound/tlv.h``.
-The available channel position values are defined in sound/asound.h,
+The available channel position values are defined in ``sound/asound.h``,
here is a cut:
-/* channel positions */
-enum {
+::
+
+ /* channel positions */
+ enum {
SNDRV_CHMAP_UNKNOWN = 0,
SNDRV_CHMAP_NA, /* N/A, silent */
SNDRV_CHMAP_MONO, /* mono stream */
SNDRV_CHMAP_TRR, /* top rear right */
SNDRV_CHMAP_TRC, /* top rear center */
SNDRV_CHMAP_LAST = SNDRV_CHMAP_TRC,
-};
+ };
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
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)
+ #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,
+``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
+When ``SNDRV_CHMAP_DRIVER_SPEC`` is set, all the channel position values
don't follow the standard definition above but driver-specific.
-* READ OPERATION
+Read Operation
+--------------
The control read operation is for providing the current channel map of
the given stream. The control element returns an integer array
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.
+``UNKNOWN``.
-* WRITE OPERATION
+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
projects / karo-tx-linux.git / commitdiff