On Fri, 9 Jan 2015 14:57:41 +0000 Russell King - ARM Linux linux@arm.linux.org.uk wrote:
Well, if we look at the HDMI specs, there is a field in the audio info frame (CA bits) which identifies the speaker allocation between the HDMI PCM channels and the physical speakers.
It describes a limited set - essentially though, channel 0 is always front left, and channel 1 is always front right. Channel 2 is always LFE, and channel 3 is always front centre.
Channel 0 and 1 are always allocated, but it's possible to have channels above 2 to be allocated independently of each other (eg, you could have 7, 6, 1, 0 allocated to front right centre, front left centre, right, left speakers - in that order.)
As you point out, we don't have documentation which tells us know how these PCM channels map to I2S inputs.
I had a look at the TDA19988 (small) datasheet I have and at the HDMI specs, and, yes, you are right: multi (stereo) channels implies speaker mapping, and, then, that the I2S streams come from a same media source.
While, as said in the TDA19988 datasheet, "The I2S-bus input interface receives an I2S-bus signal including serial data, word select and serial clock", it would be strange that these I2S buses come from different audio devices.
What we do know is that there is a fixed mapping between AP pins and I2S channels (which are not PCM channels), but as you point out, we don't have any documentation which describes how the I2S channels (each with their own L+R words) map to the PCM channels - and we don't know whether the CA_I2S bits in that same register in the TDA998x have an effect on this.
HDMI talks about LPCM (Linear PCM) channels and TDA19988 talks about I2S-bus (stereo) channels. For me, it seems obvious that these channels are correlated: - LPCM-0 is I2S-bus-0-left (FL) - LPCM-1 is I2S-bus-0-right (FR) - LPCM-2 is I2S-bus-1-left (LFE) - LPCM-3 is I2S-bus-1-right (FC) ...
Does anyone have a TDA998x setup which has an I2S source connected to the TDA998x I2S channel 1, and who has a HDMI sink which will accept the LFE/FC channels? If so, producing a description of how the CA_I2S bits and enabling I2S input pins influences the mapping would be a good idea.
I could not find a description of these CA_I2S bits.
If we don't have that, I'd recommend splitting the DT property into "audio inputs for I2S" and "audio input for SPDIF" (only one can be active with SPDIF).
If we want to support more than one SPDIF input (which must be mutually exclusive) I'd recommend to look at the OF graph stuff we use in DRM - one port for each "mode" - eg, I2S, SPDIF in on AP2, SPDIF in on AP3. Each port node can specify the AP pins which should be enabled.
I agree. There are one S/PDIF port and one I2S port.
So, which syntax?
I proposed:
audio-ports = <0x04>, <0x03>; audio-port-names = "spdif", "i2s";
Do you better like:
audio-spdif-port = <0x04>; audio-i2s-port = <0x03>;