diff options
| -rw-r--r-- | Documentation/sound/designs/channel-mapping-api.rst (renamed from Documentation/sound/alsa/Channel-Mapping-API.txt) | 77 | ||||
| -rw-r--r-- | Documentation/sound/designs/index.rst | 1 |
2 files changed, 45 insertions, 33 deletions
diff --git a/Documentation/sound/alsa/Channel-Mapping-API.txt b/Documentation/sound/designs/channel-mapping-api.rst index 3c43d1a4ca0e..58e6312a43c0 100644 --- a/Documentation/sound/alsa/Channel-Mapping-API.txt +++ b/Documentation/sound/designs/channel-mapping-api.rst @@ -1,9 +1,11 @@ +============================ 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 @@ -11,9 +13,9 @@ 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 } +``{ 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 @@ -29,8 +31,8 @@ specification. These are the main motivations for the new 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 @@ -39,10 +41,11 @@ 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 + +* 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. @@ -50,32 +53,35 @@ 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 +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 */ @@ -107,11 +113,13 @@ enum { 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 @@ -120,19 +128,21 @@ to be returned will contain such as: 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 @@ -140,9 +150,10 @@ 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. +``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 diff --git a/Documentation/sound/designs/index.rst b/Documentation/sound/designs/index.rst index 362e1c23d51f..dd4fcbcb2b92 100644 --- a/Documentation/sound/designs/index.rst +++ b/Documentation/sound/designs/index.rst @@ -4,5 +4,6 @@ Designs and Implementations .. toctree:: :maxdepth: 2 + channel-mapping-api procfile powersave |