Re: [linux-sunxi] [PATCH 05/16] ASoc: sun4i-i2s: Add 20 and 24 bit support

[Samuel Holland]

On 9/2/20 1:10 PM, Jernej Škrabec wrote:
> Hi Samuel!
> 
> Dne petek, 10. julij 2020 ob 07:44:51 CEST je Samuel Holland napisal(a):
>> On 7/4/20 6:38 AM, Clément Péron wrote:
>>> From: Marcus Cooper <codekipper@xxxxxxxxx>
>>>
>>> Extend the functionality of the driver to include support of 20 and
>>> 24 bits per sample.
>>>
>>> Signed-off-by: Marcus Cooper <codekipper@xxxxxxxxx>
>>> Signed-off-by: Clément Péron <peron.clem@xxxxxxxxx>
>>> ---
>>>
>>>  sound/soc/sunxi/sun4i-i2s.c | 11 +++++++++--
>>>  1 file changed, 9 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/sound/soc/sunxi/sun4i-i2s.c b/sound/soc/sunxi/sun4i-i2s.c
>>> index f78167e152ce..bc7f9343bc7a 100644
>>> --- a/sound/soc/sunxi/sun4i-i2s.c
>>> +++ b/sound/soc/sunxi/sun4i-i2s.c
>>> @@ -577,6 +577,9 @@ static int sun4i_i2s_hw_params(struct
>>> snd_pcm_substream *substream,> 
>>>  	case 16:
>>>  		width = DMA_SLAVE_BUSWIDTH_2_BYTES;
>>>  		break;
>>>
>>> +	case 32:
>>> +		width = DMA_SLAVE_BUSWIDTH_4_BYTES;
>>> +		break;
>>
>> This breaks the sun4i variants, because sun4i_i2s_get_wss returns 4 for a 32
>> bit width, but it needs to return 3.
> 
> I'm not sure what has WSS with physical width and DMA?

This is the change where creating a S24_LE stream no longer fails with -EINVAL.
So this is the change where userspace stops downsampling 24-bit audio sources.
So this is the change where playback of 24-bit audio sources breaks, because WSS
is programmed wrong.

>> As a side note, I wonder why we use the physical width (the spacing between
>> samples in RAM) to drive the slot width. S24_LE takes up 4 bytes per sample
>> in RAM, which we need for DMA. But I don't see why we would want to
>> transmit the padding over the wire. I would expect it to be transmitted the
>> same as S24_3LE (which has no padding). It did not matter before, because
>> the only supported format had no padding.
> 
> Allwinner DMA engines support only 1, 2, 4 and sometimes 8 bytes for bus 
> width, so if sample is 24 bits in size, we have no other way but to transmit 
> padding too.

I understand why we do 4 byte DMA from RAM <=> I2S FIFO; that was not my
question. I'm referring to the actual wire format (FIFO <=> PCM_DIN/DOUT). The
sample is already truncated from 32 bits to 24 bits in the FIFO -- that's what
TXIM and RXOM in FIFO_CTRL control.

If a sample is 24 bits wide, why would we send 32 BCLKs for every LRCK? I would
expect the slot width to match the sample resolution by default. But yet we have
this code in the driver:

    unsigned int word_size = params_width(params);
    unsigned int slot_width = params_physical_width(params);

I think slot_width should be the same as word_size, and I suggest changing it
before adding 20/24-bit support.

> Best regards,
> Jernej

Regards,
Samuel

>>>  	default:
>>>  		dev_err(dai->dev, "Unsupported physical sample width: 
> %d\n",
>>>  		
>>>  			params_physical_width(params));
>>>
>>> @@ -1063,6 +1066,10 @@ static int sun4i_i2s_dai_probe(struct snd_soc_dai
>>> *dai)> 
>>>  	return 0;
>>>  
>>>  }
>>>
>>> +#define SUN4I_FORMATS	(SNDRV_PCM_FMTBIT_S16_LE | \
>>> +			 SNDRV_PCM_FMTBIT_S20_LE | \
>>> +			 SNDRV_PCM_FMTBIT_S24_LE)
>>> +
>>>
>>>  static struct snd_soc_dai_driver sun4i_i2s_dai = {
>>>  
>>>  	.probe = sun4i_i2s_dai_probe,
>>>  	.capture = {
>>>
>>> @@ -1070,14 +1077,14 @@ static struct snd_soc_dai_driver sun4i_i2s_dai = {
>>>
>>>  		.channels_min = 1,
>>>  		.channels_max = 8,
>>>  		.rates = SNDRV_PCM_RATE_8000_192000,
>>>
>>> -		.formats = SNDRV_PCM_FMTBIT_S16_LE,
>>> +		.formats = SUN4I_FORMATS,
>>>
>>>  	},
>>>  	.playback = {
>>>  	
>>>  		.stream_name = "Playback",
>>>  		.channels_min = 1,
>>>  		.channels_max = 8,
>>>  		.rates = SNDRV_PCM_RATE_8000_192000,
>>>
>>> -		.formats = SNDRV_PCM_FMTBIT_S16_LE,
>>> +		.formats = SUN4I_FORMATS,
>>>
>>>  	},
>>>  	.ops = &sun4i_i2s_dai_ops,
>>>  	.symmetric_rates = 1,
> 
> 
> 
>