[alsa-devel] [PATCH 11/11] ALSA: digi00x: apply double-oh-three algorism to multiplex PCM samples

[Robin Gareus]

Hi Takashi,

Many thanks for doing this!

On 03/15/2015 05:01 PM, Takashi Sakamoto wrote:

> +
> +void snd_dg00x_protocol_write_s32(struct amdtp_stream *s,
> +				  struct snd_pcm_substream *pcm,
> +				  __be32 *buffer, unsigned int frames)
> +{
> +	struct snd_pcm_runtime *runtime = pcm->runtime;
> +	unsigned int channels, remaining_frames, i, c;
> +	const u32 *src;
> +	static struct dot_state state;

There's no need to declare this as static. The state is per frame.

> +	channels = s->pcm_channels;
> +	src = (void *)runtime->dma_area +
> +			frames_to_bytes(runtime, s->pcm_buffer_pointer);
> +	remaining_frames = runtime->buffer_size - s->pcm_buffer_pointer;
> +
> +	for (i = 0; i < frames; ++i) {

..it is zeroed here, anyway:

> +		dot_state_reset(&state);


> +		for (c = 0; c < channels; ++c) {
> +			buffer[s->pcm_positions[c]] =
> +					cpu_to_be32((*src >> 8) | 0x40000000);
> +			dot_encode_step(&state, &buffer[s->pcm_positions[c]]);
> +			src++;
> +		}
> +
> +		buffer += s->data_block_quadlets;
> +		if (--remaining_frames == 0)
> +			src = (void *)runtime->dma_area;
> +	}
> +}


In any case, the algorithm to xor-encode the digidesign data is not yet
100% correct there. One will need to continue iterating after the last
channel (feeding zero) until the state->carry (dot_scrt()) is 0x00.

The current code here only works only correctly for data on the first 4
chanels (18 [total channels] - 14 [max xor-chain length]).

I'll let Damien elaborate. He has access to a Digidesign interface and
did the protocol-dumps. I only did the clean room reverse-engineering
and maths on the other end.

Cheers!
robin