[alsa-devel] [PATCH v8] ASoC: bcm2835: Add 8 channel (multitrack) capability

[Matt Flax]

On 24/02/17 23:18, Matthias Reichl wrote:
> Hi Matt,
>
> please include me in CC, your emails don't seem to get through
> linux-rpi-kernel reliably.
>
> I did a few tests with WM5102 in DSP A mode connected to RPi3
> with downstream kernel 4.9.11 plus your patch. wm5102 was configured
> as master, cpu<->codec dai_link.dai_fmt =
> SND_SOC_DAIFMT_DSP_A | SND_SOC_DAIFMT_NB_NF | SND_SOC_DAIFMT_CBM_CFM
>
> Comments are inline.

Sure will do, thanks for testing. My comments are also inline.

>
> On Wed, Feb 22, 2017 at 05:56:40PM +1100, Matt Flax wrote:
>> This patch adds multitrack capability if in DSP mode A and the
>> codec is master.
>>
>> In bcm2835_i2s_startup, snd_pcm_hw_constraint_single is used to set
>> channels to 8 if both SND_SOC_DAIFMT_CBM_CFM and SND_SOC_DAIFMT_DSP_A
>> are set. Otherwise, channels are set to 2. These settings are
>> accomplished using the SNDRV_PCM_HW_PARAM_CHANNELS variable.
>>
>> In bcm2835_i2s_shutdown the channels are set to 2 by default.
>>
>> In bcm2835_i2s_hw_params, DSP mode A format is now an option.
>> Before replicating the format variable (from ch2 to ch1) for
>> register loading, requested channels are checked to be either 2 or 8.
>> This can be expanded later to accomodate other channel counts if
>> supported by the sound card hardware.
>>
>> Signed-off-by: Matt Flax <flatmax at flatmax.org>
>> ---
>>   sound/soc/bcm/bcm2835-i2s.c | 22 +++++++++++++++++-----
>>   1 file changed, 17 insertions(+), 5 deletions(-)
>>
>> diff --git a/sound/soc/bcm/bcm2835-i2s.c b/sound/soc/bcm/bcm2835-i2s.c
>> index 6ba2049..4b5f3f1 100644
>> --- a/sound/soc/bcm/bcm2835-i2s.c
>> +++ b/sound/soc/bcm/bcm2835-i2s.c
>> @@ -296,6 +296,7 @@ static int bcm2835_i2s_hw_params(struct snd_pcm_substream *substream,
>>   
>>   	switch (dev->fmt & SND_SOC_DAIFMT_FORMAT_MASK) {
>>   	case SND_SOC_DAIFMT_I2S:
>> +	case SND_SOC_DAIFMT_DSP_A:
>>   		data_delay = 1;
> In DSP_A mode data_delay should be set to 0. With data_delay = 1
> the MSB is transmitted 1 clock cycle too late and the LSB of the
> previous sample is received as MSB (i.e. you get loud noise).
>
> See these screenshots, I2S data was 0xF000 (S16_LE format)
>
> data_delay = 1:
> http://www.horus.com/~hias/tmp/rpi/bcm2835-dsp-a-delay-1.png
>
> data_delay = 0:
> http://www.horus.com/~hias/tmp/rpi/bcm2835-dsp-a-delay-0.png

I can do this, however that would require DSP mode B to have an offset 
of -1, which can't be set in the BCM2835 position register.

I really like your approach, however it needs extra resources at the 
hardware level. Without those resources it will never be usable and 
stable. The reason is that you will never be able to get proper 
synchrony and your channels will be randomly shifted. It is the curious 
nature of the BCM2835 I2S silicon - I have used an FPGA to overcome this 
problem.

>>   		break;
>>   	default:
>> @@ -312,6 +313,7 @@ static int bcm2835_i2s_hw_params(struct snd_pcm_substream *substream,
>>   
>>   	switch (params_channels(params)) {
>>   	case 2:
>> +	case 8:
>>   		format = BCM2835_I2S_CH1(format) | BCM2835_I2S_CH2(format);
>>   		format |= BCM2835_I2S_CH1(BCM2835_I2S_CHPOS(ch1pos));
>>   		format |= BCM2835_I2S_CH2(BCM2835_I2S_CHPOS(ch2pos));
>> @@ -526,7 +528,17 @@ static int bcm2835_i2s_startup(struct snd_pcm_substream *substream,
>>   	regmap_update_bits(dev->i2s_regmap, BCM2835_I2S_CS_A_REG,
>>   			BCM2835_I2S_STBY, BCM2835_I2S_STBY);
>>   
>> -	return 0;
>> +	/* Set the max channels to 8 if the codec is master and
>> +	 * we are in DSP A mode. Otherwise only allow 2 channels.
>> +	 */
>> +	if ((dev->fmt &
>> +		(SND_SOC_DAIFMT_MASTER_MASK | SND_SOC_DAIFMT_FORMAT_MASK))
>> +			== (SND_SOC_DAIFMT_CBM_CFM | SND_SOC_DAIFMT_DSP_A))
>> +		return snd_pcm_hw_constraint_single(substream->runtime,
>> +			SNDRV_PCM_HW_PARAM_CHANNELS, 8);
> I'm not sure why you are limiting to exactly 8 channels. 2 channels
> worked fine here, too. Haven't tested with 4 or 6 channels yet but I
> guess any even number of channels should work.
>
> In 8-channel configuration I often got swapped/shifted channels.
> Not 100% sure why, but probably because bcm2835 hadn't DMAed the
> data in when the codec started the clocks - in that case bcm2835 seems
> to repeat the last stereo frame data it had in it's buffer. I haven't
> digged into that deeper though, could be something in my test setup as
> well.

Ah - there you go - I would imagine they are only shifted, not swapped !

I have a very robust stream, where I never get swapped channels. In 
other words, I find that the channels 1 to 8 are always locked to the 
correct pins.

Multichannel simply can't be done (on the BCM2835) without a suitable 
FPGA/ASIC/chip between the codec and the BCM2835.

>> +	else
>> +		return snd_pcm_hw_constraint_single(substream->runtime,
>> +			SNDRV_PCM_HW_PARAM_CHANNELS, 2);
>>   }
>>   
>>   static void bcm2835_i2s_shutdown(struct snd_pcm_substream *substream,
>> @@ -549,6 +561,10 @@ static void bcm2835_i2s_shutdown(struct snd_pcm_substream *substream,
>>   	 * not stop the clock when SND_SOC_DAIFMT_CONT
>>   	 */
>>   	bcm2835_i2s_stop_clock(dev);
>> +
>> +	/* Default to 2 channels */
>> +	snd_pcm_hw_constraint_single(substream->runtime,
>> +			SNDRV_PCM_HW_PARAM_CHANNELS, 2);
>>   }
>>   
>>   static const struct snd_soc_dai_ops bcm2835_i2s_dai_ops = {
>> @@ -576,16 +592,12 @@ static struct snd_soc_dai_driver bcm2835_i2s_dai = {
>>   	.name	= "bcm2835-i2s",
>>   	.probe	= bcm2835_i2s_dai_probe,
>>   	.playback = {
>> -		.channels_min = 2,
>> -		.channels_max = 2,
>>   		.rates =	SNDRV_PCM_RATE_8000_192000,
>>   		.formats =	SNDRV_PCM_FMTBIT_S16_LE
>>   				| SNDRV_PCM_FMTBIT_S24_LE
>>   				| SNDRV_PCM_FMTBIT_S32_LE
>>   		},
>>   	.capture = {
>> -		.channels_min = 2,
>> -		.channels_max = 2,
>>   		.rates =	SNDRV_PCM_RATE_8000_192000,
>>   		.formats =	SNDRV_PCM_FMTBIT_S16_LE
>>   				| SNDRV_PCM_FMTBIT_S24_LE
> With .channels_max removed I get no alsa PCMs with the downstream
> card drivers - "aplay -L" only reported the "null" PCM.
>
> Changing that to .channels_min = 2, .channels_max = 8 brought
> back the PCMs.
>
>
That is curious, channels_max and channels_min are set in _startup 
implicitly in the snd_pcm_hw_constraint_single call which is inlined to 
the snd_pcm_hw_constraint_minmax call.

I have tested with an Audio Injector stereo card for the Pi ... I can 
confirm this problem. I will look into it further.

Can anyone explain why this would happen ?

Matt